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	$ 281,541	$ 273,657
Balance at end of period	258,178	258,178
Premium Receivable, Allowance for Credit Loss
Balance at beginning of period	156	0
Cumulative effect of adoption of updated accounting guidance for credit losses at January 1, 2020	0	156
Current period change for expected credit losses	0	0
Write-offs charged against the allowance	0	0
Balance at end of period	$ 156	$ 156