Premiums receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	$ 706,467	$ 695,277
Change in premiums receivable during period	(47,785)	11,190
Balance at end of period	658,682	706,467
Premiums in course of collection
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	84,901	73,594
Change in premiums receivable during period	10,251	11,307
Balance at end of period	95,152	84,901
Premiums accrued but unbilled
Premiums Receivable Reconciliation [Line Items]
Balance at beginning of period	621,566	621,683
Change in premiums receivable during period	(58,036)	(117)
Balance at end of period	$ 563,530	$ 621,566