Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 779,977
Due after one year through five years	1,292,316
Due after five years through ten years	812,375
Due after ten years	463,011
Amortized Cost	3,347,679	$ 3,069,645
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	784,061
Due after one year through five years	1,297,464
Due after five years through ten years	809,765
Due after ten years	478,582
Total	3,369,872	3,021,832
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	55,592
Due after one year through five years	207,221
Due after five years through ten years	138,384
Due after ten years	164,849
Amortized Cost	566,046	624,099
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	55,657
Due after one year through five years	209,710
Due after five years through ten years	142,283
Due after ten years	167,852
Total	575,502	618,672
Collateral Pledged
Investment Securities Held-to-Maturity: Amortized Cost
Amortized Cost	$ 897,000	$ 1,050,000