Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Investments measured at fair value | $ 1,174.8 | $ 2,315.3 |
| Class A | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 204,450,499 | 219,668,579 |
| Common stock, outstanding (in shares) | 204,450,499 | 219,668,579 |
| Class B | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 11,099,593 | 11,102,619 |
| Common stock, outstanding (in shares) | 11,099,593 | 11,102,619 |
| Class C | ||
| Common stock, nominal value (in dollars per share) | $ 0.01 | $ 0.01 |
| Common stock, issued (in shares) | 531,174,389 | 584,332,055 |
| Common stock, outstanding (in shares) | 531,174,389 | 584,332,055 |