Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets $ 699.6 $ 598.0 $ 628.2
Pension [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 7.1 6.3  
Pension [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 666.5 543.1  
Pension [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 26.0 48.6  
Pension [Member] | U.S. Large-cap [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 78.5 [1] 59.1 [2]  
Pension [Member] | U.S. Large-cap [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | U.S. Large-cap [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 78.5 [1] 59.1 [2]  
Pension [Member] | U.S. Large-cap [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | U.S. Mid-cap Growth [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 35.9 [1] 28.1 [2]  
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 35.9 [1] 28.1 [2]  
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | U.S. Small-cap [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 34.6 [1] 27.2 [2]  
Pension [Member] | U.S. Small-cap [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | U.S. Small-cap [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 34.6 [1] 27.2 [2]  
Pension [Member] | U.S. Small-cap [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [1] 0.0 [2]  
Pension [Member] | International [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 92.1 75.8  
Pension [Member] | International [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | International [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 92.1 75.8  
Pension [Member] | International [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Fixed Income [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 425.4 352.9  
Pension [Member] | Fixed Income [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Fixed Income [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 425.4 352.9  
Pension [Member] | Fixed Income [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Cash and Cash Equivalents [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 7.1 6.3  
Pension [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 7.1 6.3  
Pension [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Private Equity Funds [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 8.0 27.8  
Pension [Member] | Private Equity Funds [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Private Equity Funds [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Private Equity Funds [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 8.0 27.8  
Pension [Member] | Real Estate [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 18.0 20.8  
Pension [Member] | Real Estate [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Real Estate [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Pension [Member] | Real Estate [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 18.0 20.8  
Postretirement Health and Life [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 173.7 154.3 $ 171.0
Postretirement Health and Life [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 167.9 143.8  
Postretirement Health and Life [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 4.1 4.0  
Postretirement Health and Life [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 1.7 6.5  
Postretirement Health and Life [Member] | U.S. Large-cap [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 33.6 [3] 29.1 [4]  
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 33.6 [3] 29.1 [4]  
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 27.7 [3] 21.2 [4]  
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 27.7 [3] 21.2 [4]  
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | U.S. Small-cap [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 14.3 [3] 12.9 [4]  
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 14.3 [3] 12.9 [4]  
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | International [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 37.8 [3] 30.4 [4]  
Postretirement Health and Life [Member] | International [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 37.8 [3] 30.4 [4]  
Postretirement Health and Life [Member] | International [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | International [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 [3] 0.0 [4]  
Postretirement Health and Life [Member] | Mutual Funds [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 53.4 49.6  
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 53.4 49.6  
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Fixed Income [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 4.1 4.0  
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 4.1 4.0  
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 1.1 0.6  
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 1.1 0.6  
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Private Equity Funds [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 1.7 6.5  
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 1 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 2 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets 0.0 0.0  
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 3 [Member]      
Recurring Fair Value Measures [Line Items]      
Fair Value of Assets $ 1.7 $ 6.5  
[1]
The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity and fixed income securities indexes.
Recurring Fair Value Measures
 
 
Activity in Level 3
Private Equity Funds
    Real Estate
Millions
 
 
Balance as of December 31, 2018

$27.8


$20.8

Actual Return on Plan Assets
0.4

(1.3
)
Purchases, Sales, and Settlements – Net
(20.2
)
(1.5
)
Balance as of December 31, 2019

$8.0


$18.0
[2]
The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity and fixed income securities indexes.

[3] The underlying investments consist of mutual funds (Level 1).
[4] The underlying investments consist of mutual funds (Level 1).