Equity method investment - Financial Position and Results of Operations (Details) - Bankers Healthcare Group, LLC - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Assets	$ 436,693		$ 436,693		$ 330,030
Liabilities	322,323		322,323		224,837
Membership interests	114,370		114,370		105,193
Total liabilities and membership interests	436,693		436,693		$ 330,030
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenues	59,133	$ 41,997	151,937	$ 113,244
Net income	$ 30,933	$ 20,428	$ 69,039	$ 54,453