Earnings Per Share - Computation of Basic and Diluted Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2017 |
Jun. 30, 2016 |
Jun. 30, 2017 |
Jun. 30, 2016 |
|
| Numerator: | ||||
| Net income | $ 143.2 | $ 115.9 | $ 260.6 | $ 219.6 |
| Less: distributed and undistributed earnings allocated to participating securities | (1.2) | (1.2) | (2.3) | (2.3) |
| Net income attributable to common shareholders | $ 142.0 | $ 114.7 | $ 258.3 | $ 217.3 |
| Denominator: | ||||
| Weighted average basic common shares outstanding (in shares) | 93.4 | 93.2 | 93.4 | 93.6 |
| Effect of dilutive securities (in shares) | 0.2 | 0.1 | 0.2 | 0.1 |
| Weighted average diluted common shares outstanding (in shares) | 93.6 | 93.3 | 93.6 | 93.7 |
| Basic income per common share (in dollars per share) | $ 1.52 | $ 1.23 | $ 2.76 | $ 2.32 |
| Diluted income per common share (in dollars per share) | $ 1.52 | $ 1.23 | $ 2.76 | $ 2.32 |