Bank segment - Components of investment securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015 issue	Dec. 31, 2014 issue
Available-for-sale securities
Total available-for-sale securities, Amortized cost	$ 584,152	$ 549,627
Gross unrealized gains	9,178	6,745
Gross unrealized losses	(2,682)	(5,978)
Available-for-sale securities	590,648us-gaap_AvailableForSaleSecurities	550,394us-gaap_AvailableForSaleSecurities
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues	7	12
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	50,862	88,999
Gross unrealized losses, less than 12 months	(91)	(525)
Available for Sale, Securities, More Than 12 Months, Number of Issues	29	34
Fair value, 12 months or longer	166,920	201,791
Gross unrealized losses, 12 months or longer	(2,591)	(5,453)
U.S. Treasury and federal agency obligations
Available-for-sale securities
Total available-for-sale securities, Amortized cost	138,593	119,507
Gross unrealized gains	2,029	1,092
Gross unrealized losses	(395)	(1,039)
Available-for-sale securities	140,227	119,560
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues	1	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	9,973	41,970
Gross unrealized losses, less than 12 months	(2)	(361)
Available for Sale, Securities, More Than 12 Months, Number of Issues	3	5
Fair value, 12 months or longer	19,198	29,168
Gross unrealized losses, 12 months or longer	(393)	(678)
Mortgage-related securities- FNMA, FHLMC and GNMA
Available-for-sale securities
Total available-for-sale securities, Amortized cost	445,559	430,120
Gross unrealized gains	7,149	5,653
Gross unrealized losses	(2,287)	(4,939)
Available-for-sale securities	450,421	430,834
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues	6	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	40,889	47,029
Gross unrealized losses, less than 12 months	(89)	(164)
Available for Sale, Securities, More Than 12 Months, Number of Issues	26	29
Fair value, 12 months or longer	147,722	172,623
Gross unrealized losses, 12 months or longer	$ (2,198)	$ (4,775)

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

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