Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Credit Loss [Abstract]
Balance at beginning of period		$ 3,223,504	$ 2,618,175	$ 2,633,280	$ 2,064,586
Balance at end of period		3,634,182	2,866,578	3,634,182	2,866,578
Allowance for Expected Credit Losses
Balance at beginning of period		39,073	36,111	39,958	37,781
Change for provision of expected credit losses	[1]	(903)	(132)	(1,788)	(1,802)
Balance at end of period		38,170	35,979	38,170	35,979
Write-offs charged against the allowance		$ 3,300	$ 1,100	$ 4,800	$ 1,200

[1] Amounts deemed uncollectible are written-off in operating expenses. For the 2022 second quarter and 2021 second quarter, amounts written off were $3.3 million and $1.1 million, respectively. For the six months ended June 30, 2022 and 2021 period, amounts written off were $4.8 million and $1.2 million, respectively.