Stock Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Stock-Based Compensation Expense Presented in the table below is the stock-based compensation expense recorded in O&M expense in the accompanying Consolidated Statements of Operations for the years ended December 31:
 
2019
 
2018
 
2017
Stock options
$
—

 
$
1

 
$
1

RSUs and PSUs
15

 
15

 
9

Nonqualified employee stock purchase plan
2

 
1

 
1

Stock-based compensation
17

 
17

 
11

Income tax benefit
(4
)
 
(5
)
 
(4
)
Stock-based compensation expense, net of tax
$
13

 
$
12

 
$
7
Summary of Stock Option Activity
Presented in the table below is stock option activity for the year ended December 31, 2019:
 
Shares (in thousands)
 
Weighted Average Exercise Price (per share)
 
Weighted Average Remaining Life (years)
 
Aggregate Intrinsic Value
Options outstanding as of December 31, 2018
517

 
$
54.92

 
2.96
 
$
19

Granted
—

 
—

 
 
 
 
Forfeited or expired
—

 
—

 
 
 
 
Exercised
(223
)
 
52.45

 
 
 
 
Options outstanding as of December 31, 2019
294

 
$
56.80

 
2.04
 
$
19

Options exercisable as of December 31, 2019
294

 
$
56.80

 
2.04
 
$
19
Stock Options  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Additional Information to Stock Options Activity
Presented in the table below is additional information regarding stock options exercised during the years ended December 31:
 
2019
 
2018
 
2017
Intrinsic value
$
12

 
$
9

 
$
10

Exercise proceeds
14

 
7

 
11

Income tax benefit realized
3

 
2

 
3
Restricted Stock Units  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Summary of Restricted Stock Unit Activity
Presented in the table below is RSU activity for the year ended December 31, 2019:
  
Shares (in thousands)
 
Weighted Average Grant Date Fair Value (per share)
Non-vested total as of December 31, 2018
133

 
$
77.44

Granted
67

 
101.25

Vested
(65
)
 
85.27

Forfeited
(17
)
 
86.38

Non-vested total as of December 31, 2019
118

 
$
85.41
Performance Condition  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Summary of Restricted Stock Unit Activity
Presented in the table below is PSU activity for the year ended December 31, 2019:
  
Shares (in thousands)
 
Weighted Average Grant Date Fair Value (per share)
Non-vested total as of December 31, 2018
308

 
$
73.39

Granted
145

 
97.73

Vested
(112
)
 
72.84

Forfeited
(25
)
 
84.54

Non-vested total as of December 31, 2019
316

 
$
83.89
Summary of Weighted Average Assumptions Presented in the table below is the weighted average assumptions used in the Monte Carlo simulation and the weighted average grant date fair values of PSUs granted for the years ended December 31:
 
2019
 
2018
 
2017
Expected volatility
16.80
%
 
17.23
%
 
17.40
%
Risk-free interest rate
2.47
%
 
2.36
%
 
1.53
%
Expected life (years)
3.0

 
3.0

 
3.0

Grant date fair value per share
$
110.37

 
$
73.62

 
$
72.81