Investments - Restricted Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands |
Jun. 30, 2019 |
Dec. 31, 2018 |
Jun. 30, 2018 |
|---|---|---|---|
| Restricted Cash and Investments Items [Line Items] | |||
| Restricted cash | $ 381,698 | $ 130,148 | $ 130,259 |
| Restricted investments | 2,492,232 | 3,893,811 | |
| Restricted cash and investments | 2,873,930 | 4,023,959 | |
| Liability for investments purchased | 298,939 | 0 | |
| Amortized cost | 2,942,207 | 4,125,661 | |
| Third party agreements | |||
| Restricted Cash and Investments Items [Line Items] | |||
| Restricted cash | 21,299 | 21,420 | |
| Restricted investments | 69,999 | 89,596 | |
| Amortized cost | 69,855 | 88,841 | |
| Related party agreements | |||
| Restricted Cash and Investments Items [Line Items] | |||
| Restricted cash | 360,399 | 108,728 | |
| Restricted investments | 2,689,179 | 3,804,215 | |
| Amortized cost | 2,655,683 | 3,870,731 | |
| Investments Purchased for Related Party Agreements | |||
| Restricted Cash and Investments Items [Line Items] | |||
| Restricted investments – liability for investments purchased for related party agreements | $ (266,946) | $ 0 |