Fair Value Measurements - Reconciliation for Investments Measured At Fair Value on Recurring Basis Level 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	$ 321,973	$ 176,723
Purchases	90,805	304,387
Sales	(11,998)	(133,776)
Total realized and unrealized gains (losses)	970	(21,369)
Transfer into Level 3 from Level 2	27,923	46,633
Transfer out of Level 3 into Level 2	(73,005)	(50,625)
Ending fair value	356,668	321,973
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	37,386	67,178
Purchases	184	14,391
Sales	(3,520)	(65,700)
Total realized and unrealized gains (losses)	90	(57)
Transfer into Level 3 from Level 2	3,535	28,339
Transfer out of Level 3 into Level 2	(37,675)	(6,765)
Ending fair value	0	37,386
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	0	3,080
Purchases	0	0
Sales	0	(1,184)
Total realized and unrealized gains (losses)	(1)	(28)
Transfer into Level 3 from Level 2	102	1,795
Transfer out of Level 3 into Level 2	(101)	(3,663)
Ending fair value	0	0
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	7,389	21,494
Purchases	0	3,749
Sales	(784)	(5,781)
Total realized and unrealized gains (losses)	64	(645)
Transfer into Level 3 from Level 2	1,515	4,897
Transfer out of Level 3 into Level 2	(8,184)	(16,325)
Ending fair value	0	7,389
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	9,121	27,892
Purchases	0	46,074
Sales	(5,088)	(49,020)
Total realized and unrealized gains (losses)	241	(1,843)
Transfer into Level 3 from Level 2	22,771	9,890
Transfer out of Level 3 into Level 2	(27,045)	(23,872)
Ending fair value	0	9,121
Privately-held Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	228,710	0
Purchases	33,713	227,000
Sales	(2,016)	0
Total realized and unrealized gains (losses)	8,392	(2)
Transfer into Level 3 from Level 2	0	1,712
Transfer out of Level 3 into Level 2	0	0
Ending fair value	268,799	228,710
Other Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning fair value	39,367	57,079
Purchases	56,908	13,173
Sales	(590)	(12,091)
Total realized and unrealized gains (losses)	(7,816)	(18,794)
Transfer into Level 3 from Level 2	0	0
Transfer out of Level 3 into Level 2	0	0
Ending fair value	$ 87,869	$ 39,367