Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Major categories of assets measured at fair value on a recurring basis
Limited partnerships and limited liability companies (fair value)	$ 65,011	$ 66,660
Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	2,013,798	2,040,682
Total assets at fair value	2,078,809	2,107,342
Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	1,861,380	1,891,051
Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	152,418	149,631
Fixed maturity securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	2,002,420
Fixed maturity securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	1,850,002
Fixed maturity securities | Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	152,418
Fixed maturity securities | U.S. Treasury securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	43,322	49,985
Fixed maturity securities | U.S. Treasury securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	43,322	49,985
Fixed maturity securities | Corporate debt securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	1,301,687	1,338,812
Fixed maturity securities | Corporate debt securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	1,149,269	1,189,181
Fixed maturity securities | Corporate debt securities | Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	152,418	149,631
Fixed maturity securities | Government agency securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	28,589	6,531
Fixed maturity securities | Government agency securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	28,589	6,531
Fixed maturity securities | Municipal debt obligations | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	100,641	79,815
Fixed maturity securities | Municipal debt obligations | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	100,641	79,815
Fixed maturity securities | ABS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	46,020	73,582
Fixed maturity securities | ABS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	46,020	73,582
Fixed maturity securities | CLO | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	157,563	179,549
Fixed maturity securities | CLO | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	157,563	179,549
Fixed maturity securities | CMBS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	94,474	97,526
Fixed maturity securities | CMBS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	94,474	97,526
Fixed maturity securities | RMBS - non-agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	90,633	71,610
Fixed maturity securities | RMBS - non-agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	90,633	71,610
Fixed maturity securities | RMBS - agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	139,491	143,272
Fixed maturity securities | RMBS - agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	139,491	143,272
Non-redeemable preferred stock securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	11,378
Non-redeemable preferred stock securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed maturity securities	11,378
Carrying Value | Recurring
Major categories of assets measured at fair value on a recurring basis
Limited partnerships and limited liability companies (fair value)	$ 65,011	$ 66,660