Allowance for expected credit losses - Premiums receivable (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Credit Loss [Abstract]
Balance at beginning of period	$ 273,657
Balance at end of period	281,541
Allowance for Expected Credit Losses - Premiums Receivable
Balance at beginning of period	0
Cumulative effect of adoption of updated accounting guidance for credit losses at January 1, 2020	156
Current period change for expected credit losses	0
Write-offs charged against the allowance	0
Balance at end of period	$ 156