Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Mar. 31, 2021	Dec. 31, 2020
Credit Loss [Abstract]
Balance at beginning of period		$ 2,064,586	$ 1,778,717
Balance at end of period		2,618,175	2,064,586
Allowance for Expected Credit Losses
Balance at beginning of period		37,781	21,003
Cumulative effect of accounting change (1)	[1]		6,539
Change for provision of expected credit losses (1)	[2]	(1,670)	10,239
Balance at end of period		36,111	37,781
Write-offs charged against the allowance		$ 100	$ 2,800
Accounting Standards Update		us-gaap:AccountingStandardsUpdate201613Member

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

[2] Amounts deemed uncollectible are written-off in operating expenses. For the March 31, 2021 and as of December 31, 2020, amounts written off were $0.1 million and $2.8 million, respectively.