Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |
3 Months Ended | 12 Months Ended | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dec. 31, 2019 |
Sep. 30, 2019 |
Jun. 30, 2019 |
Mar. 31, 2019 |
Dec. 31, 2018 |
Sep. 30, 2018 |
Jun. 30, 2018 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Sep. 30, 2017 |
Jun. 30, 2017 |
Mar. 31, 2017 |
Dec. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Earnings Per Share - Basic [Abstract] | |||||||||||||||
| Net Income | $ 49.7 | $ 31.2 | $ 34.2 | $ 70.5 | $ 61.1 | $ 30.7 | $ 31.3 | $ 51.0 | $ 41.4 | $ 44.9 | $ 36.9 | $ 49.0 | $ 185.6 | $ 174.1 | $ 172.2 |
| Average Common Shares | 51.6 | 51.3 | 50.8 | ||||||||||||
| Earnings Per Share | $ 0.96 | $ 0.60 | $ 0.66 | $ 1.37 | $ 1.19 | $ 0.59 | $ 0.61 | $ 1.00 | $ 0.81 | $ 0.88 | $ 0.73 | $ 0.97 | $ 3.59 | $ 3.39 | $ 3.39 |
| Earnings Per Share - Diluted [Abstract] | |||||||||||||||
| Net Income (Loss) Available to Common Stockholders, Diluted | $ 185.6 | $ 174.1 | $ 172.2 | ||||||||||||
| Average Common Shares | 51.7 | 51.5 | 51.0 | ||||||||||||
| Earnings Per Share | $ 0.96 | $ 0.60 | $ 0.66 | $ 1.37 | $ 1.18 | $ 0.59 | $ 0.61 | $ 0.99 | $ 0.81 | $ 0.88 | $ 0.72 | $ 0.97 | $ 3.59 | $ 3.38 | $ 3.38 |
| Dilutive Securities (Shares) | 0.1 | 0.2 | 0.2 | ||||||||||||