Investments - Narrative (Details) - USD ($) $ in Thousands |
6 Months Ended | ||
|---|---|---|---|
Jun. 30, 2017 |
Jun. 30, 2018 |
Dec. 31, 2017 |
|
| Schedule of Available-for-sale Securities [Line Items] | |||
| Unrealized loss position, accumulated loss | $ 14,906 | $ 3,723 | |
| Unrealized loss position, 12 months or greater | 4,707 | 2,173 | |
| Other-than-temporary impairment | $ 144 | ||
| U.S. Treasury securities and obligations of U.S. government agencies | |||
| Schedule of Available-for-sale Securities [Line Items] | |||
| Cash and investments held with various state insurance departments | 5,200 | 7,000 | |
| Unrealized loss position, accumulated loss | 2,060 | 981 | |
| Unrealized loss position, 12 months or greater | $ 873 | $ 587 |