Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 371,218
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,227,734
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	228,192
Available-for-Sale Amortized Cost, After Ten Years	248,970
Available-for-Sale, Amortized Cost	2,076,114	$ 1,624,976
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	372,541
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,229,254
Available-for-Sale Estimated Fair Value, After Ten Years	228,561
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	249,838
Available-for-Sale, Estimated Fair Value	2,080,194	1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	75,748
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	268,855
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	133,245
Held-to-Maturity Amortized Cost, After Ten Years	51,594
Held-to-Maturity, Amortized Cost	529,442	512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	74,416
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	280,504
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	134,858
Held-to-Maturity Estimated Fair Value, After Ten Years	45,367
Held-to-Maturity, Estimated Fair Value	535,145	$ 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	63,743
Investment Securities Primarily Classified as Available-for-Sale, Fair Value, After One Year but Within Five Years	$ 63,933