Equity method investment - Financial Position and Results of Operations (Details) - Bankers Healthcare Group, LLC - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Assets	$ 253,355		$ 253,355		$ 223,246
Liabilities	169,357		169,357		139,531
Membership interests	83,998		83,998		83,715
Total liabilities and membership	253,355		253,355		$ 223,246
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenues	37,012	$ 39,330	71,247	$ 70,618
Net income	$ 18,013	$ 21,439	$ 34,024	$ 33,593