Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 345,833
Due after one year through five years	731,523
Due after five years through ten years	488,832
Due after ten years	213,558
Preferred stock	1,389
Total	1,781,135
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	344,064
Due after one year through five years	726,231
Due after five years through ten years	485,107
Due after ten years	210,229
Preferred stock	1,847
Total	1,767,478
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	85,434
Due after one year through five years	253,002
Due after five years through ten years	143,396
Due after ten years	163,773
Total	645,605	$ 623,427
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	85,398
Due after one year through five years	251,488
Due after five years through ten years	141,140
Due after ten years	162,017
Total	640,043	608,531
Securities with amortized cost pledged to secure borrowings and deposits	$ 1,050,000	$ 794,000