Investments - Summary of unrealized gains and losses (Details) - security |
Sep. 30, 2016 |
Dec. 31, 2015 |
|---|---|---|
| Investments, Debt and Equity Securities [Abstract] | ||
| Number of positions held with unrealized gains (positions) | 1,057 | 602 |
| Number of positions held with unrealized losses (positions) | 172 | 561 |
| Number of positions held that individually exceed unrealized gains of $500,000 (positions) | 2.00 | 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) | 92.00% | 94.00% |
| Percentage of positions held with unrealized losses that were investment grade (percent) | 88.00% | 89.00% |
| Percentage of fair value held with unrealized gains that were investment grade (percent) | 92.00% | 95.00% |
| Percentage of fair value held with unrealized losses that were investment grade (percent) | 91.00% | 88.00% |