Equity method investment Equity method investment - Financial Position and Results of Operations (Details) - Bankers Healthcare Group, LLC - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Assets	$ 840,398,000	$ 459,816,000
Liabilities	641,037,000	324,211,000
Equity interests	199,361,000	135,605,000
Total liabilities and equity interests	840,398,000	459,816,000
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenues	366,500,000	220,253,000	$ 160,209,000
Net income, pre tax	$ 182,462,000	$ 104,297,000	$ 77,941,000