Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 37,082
Due after one year through five years	116,724
Due after five years through ten years	402,343
Due after ten years	2,026,509
Amortized Cost	2,582,658	$ 1,976,646
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	36,851
Due after one year through five years	114,934
Due after five years through ten years	391,936
Due after ten years	1,986,189
Total	2,529,910	1,963,546
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	57,341
Due after one year through five years	222,454
Due after five years through ten years	140,431
Due after ten years	182,461
Amortized Cost	602,687	677,093
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	57,143
Due after one year through five years	219,379
Due after five years through ten years	136,972
Due after ten years	178,804
Total	592,298	662,906
Collateral Pledged
Investment Securities Held-to-Maturity: Amortized Cost
Amortized Cost	$ 967,900	$ 937,200