Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Total	$ 1,408,996	$ 1,441,724
Fair Value
Total	1,391,862	1,428,617
Debt securities
Amortized Cost
Due in one year or less	51,909	17,878
Due after one year through five years	368,846	376,777
Due after five years through ten years	162,061	210,985
Due after ten years	18,591	13,827
Total	601,407	619,467
Fair Value
Due in one year or less	51,932	18,034
Due after one year through five years	367,377	378,631
Due after five years through ten years	160,374	208,999
Due after ten years	18,191	13,181
Total	597,874	618,845
Mortgage-backed securities
Amortized Cost
Total	807,589	822,257
Fair Value
Total	$ 793,988	$ 809,772