Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-Sale Amortized Cost, Within One Year	$ 458,800
Available-for-Sale Amortized Cost, After One Year but Within Five Years	1,490,200
Available-for-Sale Amortized Cost, After Five Years but Within Ten Years	218,200
Available-for-Sale Amortized Cost, After Ten Years	59,000
Amortized Cost	2,226,200	$ 2,229,300
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-Sale Estimated Fair Value, Within One Year	450,500
Available-for-Sale Estimated Fair Value, After One Year but Within Five Years	1,457,500
Available-for-Sale Estimated Fair Value, After Five Years but Within Ten Years	212,200
Available-for-Sale Estimated Fair Value, After Ten Years	57,800
Available-for-Sale, Estimated Fair Value	2,178,000	2,208,700
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-Maturity Amortized Cost, Within One Year	76,300
Held-to-Maturity Amortized Cost, After One Year but Within Five Years	221,400
Held-to-Maturity Amortized Cost, After Five Years but Within Ten Years	110,700
Held-to-Maturity Amortized Cost, After Ten Years	32,500
Amortized Cost	440,900	484,500
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-Maturity Estimated Fair Value, Within One Year	75,300
Held-to-Maturity Estimated Fair Value, After One Year but Within Five Years	215,700
Held-to-Maturity Estimated Fair Value, After Five Years but Within Ten Years	110,500
Held-to-Maturity Estimated Fair Value, After Ten Years	31,300
Held-to-Maturity, Estimated Fair Value	432,800	$ 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	107,200
Fair value of investment securities callable after one year but within five years	$ 104,800