Premiums receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	$ 658,682	$ 706,467
Change in premiums receivable during period	66,708	(47,785)
Balance at end of period	725,390	658,682
Premiums in course of collection
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	95,152	84,901
Change in premiums receivable during period	6,250	10,251
Balance at end of period	101,402	95,152
Premiums accrued but unbilled
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	563,530	621,566
Change in premiums receivable during period	60,458	(58,036)
Balance at end of period	$ 623,988	$ 563,530