Investments - Net Investment Income (Details) - USD ($) $ in Thousands |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2017 |
Jun. 30, 2016 |
Jun. 30, 2017 |
Jun. 30, 2016 |
|
| Investment Income, Net [Abstract] | ||||
| Investment income | $ 4,099 | $ 3,536 | $ 8,092 | $ 6,945 |
| Investment expenses | (191) | (194) | (377) | (372) |
| Net investment income | $ 3,908 | $ 3,342 | $ 7,715 | $ 6,573 |