Equity method investment - Narrative (Details) - USD ($)	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]
Technology, trade name and customer relationship intangibles	$ 48,737,000		$ 48,737,000		$ 56,710,000
Amortization of Intangible Assets	2,616,000	$ 3,077,000	7,973,000	$ 5,745,000
Dividends received from equity method investment			33,651,000	19,372,000
Bankers Healthcare Group, LLC
Schedule of Equity Method Investments [Line Items]
Technology, trade name and customer relationship intangibles	11,300,000		11,300,000		$ 13,400,000
Amortization of Intangible Assets	693,000	832,000	2,100,000	2,500,000
Accretion income	719,000	758,000	2,200,000	2,300,000
Dividends received from equity method investment	10,200,000	4,500,000	33,700,000	19,400,000
Loan face amount	$ 0	$ 0	$ 0	$ 0