Securities (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2014 security	Dec. 31, 2013 security	Dec. 31, 2012
Securities
Number of securities held with unrealized loss	80	109
Proceeds from sales of securities available-for-sale	$ 14,447,000	$ 41,640,000	$ 60,318,000
Gross realized gains from sale of available for sale securities	141,000	2,057,000	8,477,000
Total available-for-sale securities	384,359,000	401,690,000
Held-to-maturity, amortized cost	2,133,804,000	1,849,526,000
Held-to-maturity, fair value	2,131,371,000	1,811,122,000
Callable securities, amortized cost	218,000,000
Callable securities amortized cost, callable within one year and thereafter	201,600,000
Accrued interest receivable for securities	6,700,000	6,300,000
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	98,700,000
Total available-for-sale securities	96,700,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity, amortized cost	119,900,000
Held-to-maturity, fair value	$ 118,400,000