Allowance for Expected Credit Losses - Reinsurance recoverables (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Credit Loss [Abstract]
Balance at beginning of period		$ 4,041,076	$ 4,303,135	$ 4,500,802	$ 4,346,816
Balance at end of period		4,314,515	4,363,507	4,314,515	4,363,507
Allowance for Expected Credit Losses
Balance at beginning of period		10,872	13,700	11,636	1,364
Cumulative effect of accounting change	[1]				12,010
Change for provision of expected credit losses		157	(105)	(607)	221
Balance at end of period		$ 11,029	$ 13,595	$ 11,029	$ 13,595

[1] Adoption of ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326)”