Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($)
shares in Thousands, $ in Millions |
12 Months Ended |
|
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Summary of Common Stock [Line Items] |
|
|
|
|
| Balance, Shares |
51,700
|
51,500
|
|
|
| Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |
$ 0.0
|
$ 0.0
|
$ 13.5
|
|
| Common Stock [Member] |
|
|
|
|
| Summary of Common Stock [Line Items] |
|
|
|
|
| Balance, Shares |
51,679
|
51,509
|
51,117
|
49,560
|
| Balance, Equity |
$ 1,436.7
|
$ 1,428.5
|
$ 1,401.4
|
$ 1,295.3
|
| Employee Stock Purchase Plan, Shares |
8
|
11
|
12
|
|
| Employee Stock Purchase Plan, Equity |
$ 0.7
|
$ 0.8
|
$ 0.8
|
|
| Invest Direct, Shares |
38
|
277
|
257
|
|
| Invest Direct, Equity |
$ 3.0
|
$ 20.7
|
$ 19.0
|
|
| Options and Stock Awards, Shares |
85
|
57
|
22
|
|
| Options and Stock Awards, Equity |
$ 1.3
|
$ 2.1
|
$ 3.6
|
|
| Contributions to RSOP, Shares |
39
|
47
|
50
|
|
| Contributions to RSOP, Equity |
$ 3.2
|
$ 3.5
|
$ 3.5
|
|
| Equity Issuance Program, Shares |
0
|
0
|
1,000
|
|
| Equity Issuance Program, Equity |
|
|
$ 65.7
|
|
| Employer Contributions to Pension - Shares |
0
|
0
|
200
|
|
| Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |
|
|
$ 13.5
|
|
| Equity Issuance Program Shares Authorized |
|
|
13,600
|
|
| Equity Issuance Program Shares Available for Issuance |
2,900
|
|
|
|
| Pension Plan [Member] |
|
|
|
|
| Summary of Common Stock [Line Items] |
|
|
|
|
| Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |
$ 0.0
|
|
$ 0.0
|
|
| Pension Plan [Member] | Common Stock [Member] |
|
|
|
|
| Summary of Common Stock [Line Items] |
|
|
|
|
| Employer Contributions to Pension - Shares |
|
|
216
|
|
| Employer Contributions to Defined Benefit Pension Plans - Common Stock Value |
|
|
$ 13.5
|
|