Allowance for Expected Credit Losses - Premiums receivable (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Credit Loss [Abstract]
Balance at beginning of period	$ 1,778,717
Balance at end of period	2,155,204
Allowance for Expected Credit Losses
Balance at beginning of period	21,003
Cumulative effect of accounting change (1)	6,539
Change for provision for expected credit losses	448
Write-offs charged against the allowance	0
Balance at end of period	$ 27,990