Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 433,000
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,445,700
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	278,800
Available-for-Sale Amortized Cost, After Ten Years	167,500
Amortized Cost	2,325,000	$ 2,229,300
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	426,300
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,417,800
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	272,200
Available-for-Sale Estimated Fair Value, After Ten Years	164,400
Available-for-Sale, Estimated Fair Value	2,280,700	2,208,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	61,400
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	247,200
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	121,000
Held-to-Maturity Amortized Cost, After Ten Years	32,700
Amortized Cost	462,300	484,500
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	61,200
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	241,600
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	120,800
Held-to-Maturity Estimated Fair Value, After Ten Years	31,900
Held-to-Maturity, Estimated Fair Value	455,500	$ 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	147,500
Fair value of investment securities callable after one year but within five years	$ 144,400