Investments - Summary of unrealized gains and losses (Details) - security |
Mar. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Investments, Debt and Equity Securities [Abstract] | ||
| Number of positions held with unrealized gains (positions) | 302 | 496 |
| Number of positions held with unrealized losses (positions) | 996 | 722 |
| Number of positions held that individually exceed unrealized gains of $500,000 (positions) | 0 | 2 |
| Number of positions held that individually exceed unrealized losses of $500,000 (positions) | 0 | 0 |
| Percentage of positions held with unrealized gains that were investment grade (percent) | 84.00% | 81.00% |
| Percentage of positions held with unrealized losses that were investment grade (percent) | 94.00% | 97.00% |
| Percentage of fair value held with unrealized gains that were investment grade (percent) | 88.00% | 81.00% |
| Percentage of fair value held with unrealized losses that were investment grade (percent) | 93.00% | 95.00% |