Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Credit Loss [Abstract]
Balance at beginning of period		$ 2,633,280	$ 2,064,586
Balance at end of period		3,223,504	2,618,175
Allowance for Expected Credit Losses
Balance at beginning of period		39,958	37,781
Change for provision of expected credit losses	[1]	(885)	(1,670)
Balance at end of period		39,073	36,111
Write-offs charged against the allowance		$ 1,500	$ 100

[1] Amounts deemed uncollectible are written-off in operating expenses. For the 2022 first quarter and 2021 first quarter, amounts written off were $1.5 million and $0.1 million, respectively.