Securities (Narrative) (Details) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($) security	Jun. 30, 2015 USD ($) security	Dec. 31, 2014 USD ($) security
Securities
Number of securities held with unrealized loss | security	94	94	80
Total available-for-sale securities	$ 428,745	$ 428,745	$ 384,359
Held-to-maturity Securities	2,156,882	2,156,882	2,133,804
Held-to-maturity securities, fair value	2,152,091	2,152,091	2,131,371
Callable securities, amortized cost	306,000	306,000
Callable securities amortized cost, callable within one year and thereafter	289,600	289,600
Accrued interest receivable for securities	7,100	7,100	$ 6,700
Proceeds from sale of available-for-sale securities		19,026
Available-for-sale securities, gross realized gains	72	72
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	98,700	98,700
Total available-for-sale securities	96,000	96,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity Securities	207,800	207,800
Held-to-maturity securities, fair value	$ 204,900	$ 204,900