Bank segment - Components of investment securities (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) issue	Dec. 31, 2014 USD ($) issue
Available-for-sale securities
Total available-for-sale securities, Amortized cost	$ 693,157	$ 549,627
Gross unrealized gains	6,572	6,745
Gross unrealized losses	(6,209)	(5,978)
Available-for-sale securities	$ 693,520	$ 550,394
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	32	12
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 247,800	$ 88,999
Gross unrealized losses, less than 12 months	$ (2,099)	$ (525)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	28	34
Fair value, 12 months or longer	$ 154,878	$ 201,791
Gross unrealized losses, 12 months or longer	(4,110)	(5,453)
U.S. Treasury and federal agency obligations
Available-for-sale securities
Total available-for-sale securities, Amortized cost	168,143	119,507
Gross unrealized gains	1,355	1,092
Gross unrealized losses	(1,039)	(1,039)
Available-for-sale securities	$ 168,459	$ 119,560
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	12	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 76,835	$ 41,970
Gross unrealized losses, less than 12 months	$ (477)	$ (361)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	3	5
Fair value, 12 months or longer	$ 18,332	$ 29,168
Gross unrealized losses, 12 months or longer	(562)	(678)
Mortgage-related securities- FNMA, FHLMC and GNMA
Available-for-sale securities
Total available-for-sale securities, Amortized cost	525,014	430,120
Gross unrealized gains	5,217	5,653
Gross unrealized losses	(5,170)	(4,939)
Available-for-sale securities	$ 525,061	$ 430,834
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	20	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 170,965	$ 47,029
Gross unrealized losses, less than 12 months	$ (1,622)	$ (164)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	25	29
Fair value, 12 months or longer	$ 136,546	$ 172,623
Gross unrealized losses, 12 months or longer	$ (3,548)	$ (4,775)