Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2016 USD ($) security	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) security
Securities
Number of securities held with unrealized loss | security	61		129
Proceeds from sales of securities available-for-sale	$ 23,065,000	$ 0
Gain on sales of securities	86,000	$ 0
Total available-for-sale securities	388,878,000		$ 416,798,000
Held-to-maturity securities	2,443,503,000		2,296,799,000
Held-to-maturity securities, fair value	2,469,458,000		2,286,092,000
Callable securities, amortized cost	211,300,000
Callable securities amortized cost, callable within one year and thereafter	194,900,000
Accrued interest receivable for securities	7,200,000		$ 7,400,000
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	73,700,000
Total available-for-sale securities	73,700,000
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity securities	273,000,000
Held-to-maturity securities, fair value	$ 271,100,000