Securities (Narrative) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) security	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) security	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Number of securities held with unrealized loss | security	55		55		129
Proceeds from sales of securities available-for-sale			$ 23,065	$ 19,026
Gain on sales of securities	$ 0	$ 72	86	$ 72
Securities
Available-for-sale, estimated fair value	356,347		356,347		$ 416,798
Held-to-maturity securities, amortized cost	2,677,930		2,677,930		2,296,799
Held-to-maturity securities, estimated fair value	2,716,702		2,716,702		2,286,092
Callable securities, amortized cost	251,300		251,300
Callable securities amortized cost, callable within one year and thereafter	234,900		234,900
Accrued interest receivable for securities	7,400		7,400		$ 7,400
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	55,000		55,000
Available-for-sale, estimated fair value	55,000		55,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity securities, amortized cost	326,700		326,700
Held-to-maturity securities, estimated fair value	$ 325,900		$ 325,900