Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Thousands		3 Months Ended	9 Months Ended
		Sep. 30, 2020	Sep. 30, 2020
Credit Loss [Abstract]
Balance at beginning of period		$ 2,203,753	$ 1,778,717
Balance at end of period		2,225,311	2,225,311
Allowance for Expected Credit Losses
Balance at beginning of period		36,054	21,003
Cumulative effect of accounting change (1)	[1]	0	6,539
Change for provision of expected credit losses (2)	[2]	1,046	9,558
Balance at end of period		37,100	37,100
Write-offs charged against the allowance		$ 0	$ 2,300

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

[2] Amounts deemed uncollectible are written-off in operating expenses. For the 2020 third quarter and nine months ended September 30, 2020, amounts written off totaled nil and $2.3 million, respectively.