Equity method investment Equity method investment - Financial Position and Results of Operations (Details) - Bankers Healthcare Group, LLC [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Assets	$ 330,030,000	$ 223,246,000
Liabilities	224,837,000	139,531,000
Equity interests	105,193,000	83,715,000
Total liabilities and equity interests	330,030,000	223,246,000
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenues	160,209,000	136,693,000
Net income, pre tax	$ 77,941,000	$ 67,135,000