Allowance for Expected Credit Losses - Reinsurance recoverables (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Credit Loss [Abstract]
Balance at beginning of period	$ 4,346,816
Balance at end of period	4,303,135
Allowance for Expected Credit Losses
Balance at beginning of period	1,364
Cumulative effect of accounting change (1)	12,010
Additions for current-period provision for expected credit losses	326
Write-offs charged against the allowance	0
Balance at end of period	$ 13,700