Securities - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Investments, Debt and Equity Securities [Abstract]
Number of securities which had an unrealized loss | security	188	129
Proceeds from sales of securities from the available-for-sale portfolio	$ 23,065	$ 19,026	$ 14,447
Gross realized gains	86	72	$ 141
Securities
Available-for-sale, amortized cost	282,200
Available-for-sale, estimated fair value	280,045	416,798
Held-to-maturity, amortized cost	2,740,132	2,296,799
Held-to-maturity, estimated fair value	2,690,546	2,286,092
Amortized cost of callable securities	374,300
Accrued interest receivable for securities	8,100	$ 7,400
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale, amortized cost	30,000
Available-for-sale, estimated fair value	28,900
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity, amortized cost	464,700
Held-to-maturity, estimated fair value	$ 443,700