Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Major categories of assets measured at fair value on a recurring basis
Limited partnerships and limited liability companies (fair value)	$ 66,660	$ 53,432
Transfers from Level 1 to Level 2	0	0
Transfers from Level 2 to Level 3		126,500
Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	2,040,682	1,693,382
Limited partnerships and limited liability companies (fair value)	66,660	53,432
Total assets at fair value	2,107,342	1,746,814
Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	1,891,051	1,566,885
Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	149,631	126,497
U.S. Treasury securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	49,985	90,328
U.S. Treasury securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	49,985	90,328
Corporate debt securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	1,338,812	1,192,430
Corporate debt securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	1,189,181	1,065,933
Corporate debt securities | Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	149,631	126,497
Government agency securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	6,531
Government agency securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	6,531
Municipal debt obligations | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	79,815	6,085
Municipal debt obligations | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	79,815	6,085
ABS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	73,582	83,581
ABS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	73,582	83,581
CLO | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	179,549	156,913
CLO | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	179,549	156,913
CMBS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	97,526	53,843
CMBS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	97,526	53,843
RMBS - non-agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	71,610	79,551
RMBS - non-agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	71,610	79,551
RMBS - agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	143,272	30,651
RMBS - agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities	$ 143,272	$ 30,651