Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Investment1 [Line Items]
Total available-for-sale	$ 2,346,267	$ 2,061,467
Allowance for Credit Losses	(441)	(397)
Gross Unrealized Gains	80,952	138,110
Gross Unrealized Losses	(16,891)	(2,554)
Fair Value	2,409,887	2,196,626
U.S. government
Investment1 [Line Items]
Total available-for-sale	127,752	170,110
Gross Unrealized Gains	6,846	13,504
Gross Unrealized Losses	(44)	(257)
Fair Value	134,554	183,357
U.S. Agency
Investment1 [Line Items]
Total available-for-sale	30,403	28,902
Gross Unrealized Gains	2,374	3,970
Gross Unrealized Losses	(17)
Fair Value	32,760	32,872
Non-U.S. govt. & agency
Investment1 [Line Items]
Total available-for-sale	8,297	10,298
Gross Unrealized Gains	338	667
Gross Unrealized Losses	(154)
Fair Value	8,481	10,965
Agency MBS
Investment1 [Line Items]
Total available-for-sale	362,861	384,015
Gross Unrealized Gains	9,277	18,789
Gross Unrealized Losses	(4,951)	(733)
Fair Value	367,187	402,071
ABS/CMBS/MBS
Investment1 [Line Items]
Total available-for-sale	264,273	213,223
Allowance for Credit Losses		(17)
Gross Unrealized Gains	2,120	5,580
Gross Unrealized Losses	(2,339)	(413)
Fair Value	264,054	218,373
Corporate Debt
Investment1 [Line Items]
Total available-for-sale	925,394	753,404
Allowance for Credit Losses	(441)	(380)
Gross Unrealized Gains	37,247	64,501
Gross Unrealized Losses	(5,105)	(933)
Fair Value	957,095	816,592
Municipal
Investment1 [Line Items]
Total available-for-sale	627,287	501,515
Gross Unrealized Gains	22,750	31,099
Gross Unrealized Losses	(4,281)	(218)
Fair Value	$ 645,756	$ 532,396