Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020
Credit Loss [Abstract]
Balance at beginning of period		$ 2,155,204	$ 1,778,717
Balance at end of period		2,203,753	2,203,753
Allowance for Expected Credit Losses
Balance at beginning of period		27,990	21,003
Cumulative effect of accounting change (1)	[1]	0	6,539
Change for provision of expected credit losses (2)	[2]	8,064	8,512
Balance at end of period		36,054	36,054
Write-offs charged against the allowance		$ 1,800	$ 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 second quarter and six months ended June 30, 2020, amounts written off totaled $1.8 million and $2.3 million, respectively.