Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 21,498
Due after one year through five years	131,011
Due after five years through ten years	397,319
Due after ten years	1,786,629
Amortized Cost	2,337,846	$ 1,976,646
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	21,471
Due after one year through five years	129,135
Due after five years through ten years	388,757
Due after ten years	1,755,953
Total	2,297,123	1,963,546
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	81,162
Due after one year through five years	241,937
Due after five years through ten years	162,648
Due after ten years	191,100
Preferred stock, amortized cost	1,389
Amortized Cost	676,847	677,093
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	81,076
Due after one year through five years	238,254
Due after five years through ten years	157,828
Due after ten years	186,463
Preferred stock, fair value	1,807
Total	663,621	662,906
Securities with amortized cost pledged to secure borrowings and deposits	$ 1,060,000	$ 937,200