Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 326,690
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,108,186
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	164,443
Available-for-Sale Amortized Cost, After Ten Years	62,193
Available-for-Sale, Amortized Cost	1,661,512	$ 1,624,976
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	326,645
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,105,388
Available-for-Sale Estimated Fair Value, After Ten Years	163,370
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	62,188
Available-for-Sale, Estimated Fair Value	1,657,591	1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	75,598
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	278,753
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	109,051
Held-to-Maturity Amortized Cost, After Ten Years	27,566
Held-to-Maturity, Amortized Cost	490,968	512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	76,786
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	278,372
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	110,987
Held-to-Maturity Estimated Fair Value, After Ten Years	27,780
Held-to-Maturity, Estimated Fair Value	493,925	$ 513,273
Callable Within One Year
Available-for-sale Securities and Held-to-maturity Securities
Investment Securities Primarily Classified as Available-for-Sale, Amortized Costs, After One Year but Within Five Years	178,739
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 176,310