Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Sep. 30, 2019
Major categories of assets measured at fair value on a recurring basis
Investments in limited partnership and limited liability companies		$ 53,432	$ 65,322
Transfers from level 1 to level 2		0	0
Transfer 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		1,693,382	2,025,955
Total assets at fair value		1,746,814	2,091,277
Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		1,566,885	1,896,369
Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		126,497	129,586
U.S. Treasury securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		90,328	73,351
U.S. Treasury securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		90,328	73,351
Government agency securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities			4,846
Government agency securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities			4,846
Corporate debt securities
Major categories of assets measured at fair value on a recurring basis
Transfer from level 2 to level 3	$ 119,004
Corporate debt securities | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		1,192,430	1,375,782
Corporate debt securities | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		1,065,933	1,246,196
Corporate debt securities | Level 3 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		126,497	129,586
Municipal debt obligations | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		6,085	14,756
Municipal debt obligations | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		6,085	14,756
ABS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		83,581	85,216
ABS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		83,581	85,216
CLO | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		156,913	179,744
CLO | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		156,913	179,744
CMBS | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		53,843	70,360
CMBS | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		53,843	70,360
RMBS - non-agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		79,551	74,314
RMBS - non-agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		79,551	74,314
RMBS - agency | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		30,651	147,586
RMBS - agency | Level 2 | Recurring
Major categories of assets measured at fair value on a recurring basis
Total fixed income securities		30,651	147,586
Carrying Value | Recurring
Major categories of assets measured at fair value on a recurring basis
Investments in limited partnership and limited liability companies		$ 53,432	$ 65,322