Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 32,586
Due after one year through five years	121,421
Due after five years through ten years	455,582
Due after ten years	2,196,692
Amortized Cost	2,806,281	$ 1,976,646
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	32,424
Due after one year through five years	119,478
Due after five years through ten years	443,027
Due after ten years	2,141,951
Total	2,736,880	1,963,546
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	57,403
Due after one year through five years	221,335
Due after five years through ten years	157,517
Due after ten years	172,112
Amortized Cost	608,367	677,093
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	57,248
Due after one year through five years	218,555
Due after five years through ten years	153,997
Due after ten years	168,397
Total	598,197	662,906
Collateral Pledged
Investment Securities Held-to-Maturity: Amortized Cost
Amortized Cost	$ 1,020,000	$ 937,200