Securities - Narrative (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) security	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) security
Investments, Debt and Equity Securities [Abstract]
Number of securities held with unrealized loss | security	195		188
Proceeds from sales of securities from available-for-sale portfolio	$ 0	$ 23,065
Gross realized gains	0	$ 86
Securities
Available-for-sale debt securities, amortized cost	268,762
Available-for-sale securities, estimated fair value	265,899		$ 280,045
Held-to-maturity securities, amortized cost	2,769,376		2,740,132
Held-to-maturity securities, estimated fair value	2,721,723		2,690,546
Callable securities, amortized cost	374,300
Accrued interest receivable for securities	8,300		$ 8,100
Available-for-sale debt securities, excluding mortgage-backed securities
Securities
Available-for-sale debt securities, amortized cost	30,000
Available-for-sale securities, estimated fair value	28,900
Held-to-maturity debt securities, excluding mortgage-backed securities
Securities
Held-to-maturity securities, amortized cost	459,600
Held-to-maturity securities, estimated fair value	$ 439,200