Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 375,199
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,287,854
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	278,782
Available-for-Sale Amortized Cost, After Ten Years	168,365
Amortized Cost	2,110,200	$ 1,624,976
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	376,264
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,288,384
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	278,277
Available-for-Sale Estimated Fair Value, After Ten Years	168,772
Available-for-Sale, Estimated Fair Value	2,111,697	1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	87,077
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	264,417
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	119,579
Held-to-Maturity Amortized Cost, After Ten Years	34,912
Amortized Cost	505,985	512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	83,899
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	272,442
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	121,039
Held-to-Maturity Estimated Fair Value, After Ten Years	33,025
Held-to-Maturity, Estimated Fair Value	510,405	$ 513,273
Callable Within One Year
Available-for-sale Securities and Held-to-maturity Securities
Amortized cost of investment securities callable after one year but within five years	94,461
Fair value of investment securities callable after one year but within five years	$ 94,348