Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 511,500
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,444,900
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	189,300
Available-for-Sale Amortized Cost, After Ten Years	67,500
Amortized Cost	2,213,200	$ 2,229,300
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	501,800
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,407,900
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	183,300
Available-for-Sale Estimated Fair Value, After Ten Years	65,800
Available-for-Sale, Estimated Fair Value	2,158,800	2,208,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	70,600
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	222,900
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	100,700
Held-to-Maturity Amortized Cost, After Ten Years	23,600
Amortized Cost	417,800	484,500
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	68,900
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	216,000
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	99,900
Held-to-Maturity Estimated Fair Value, After Ten Years	22,600
Held-to-Maturity, Estimated Fair Value	407,400	$ 483,300
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	107,400
Fair value of investment securities callable after one year but within five years	$ 105,100