Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) - USD ($) $ in Millions |
Jun. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Investments | $ 2,164.1 | $ 2,315.3 |
| Class A | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 207,403,209 | 219,668,579 |
| Common stock, outstanding (in shares) | 207,403,209 | 219,668,579 |
| Class B | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 11,102,619 | 11,102,619 |
| Common stock, outstanding (in shares) | 11,102,619 | 11,102,619 |
| Class C | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 555,820,059 | 584,332,055 |
| Common stock, outstanding (in shares) | 555,820,059 | 584,332,055 |