Investment Securities - Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investment Securities Available-for-Sale: Amortized Cost
Due in one year or less	$ 370,316
Due after one year through five years	794,328
Due after five years through ten years	578,043
Due after ten years	297,878
Preferred stock	1,389
Total	2,041,954
Investment Securities Available-for-Sale: Fair Value
Due in one year or less	368,404
Due after one year through five years	789,726
Due after five years through ten years	574,481
Due after ten years	295,189
Preferred stock	1,872
Total	2,029,672
Investment Securities Held-to-Maturity: Amortized Cost
Due in one year or less	85,395
Due after one year through five years	249,160
Due after five years through ten years	149,527
Due after ten years	173,094
Total	657,176	$ 623,427
Investment Securities Held-to-Maturity: Fair Value
Due in one year or less	85,413
Due after one year through five years	249,315
Due after five years through ten years	149,102
Due after ten years	172,129
Total	655,959	608,531
Securities with amortized cost pledged to secure borrowings and deposits	$ 963,200	$ 794,000