Securities - Available for Sale Securities Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Amortized Cost	$ 1,360,508	$ 1,408,996
Fair Value
Total	1,310,672	1,391,863
Total available-for-sale securities
Amortized Cost
Due in one year or less	87,756	51,909
Due after one year through five years	320,407	368,846
Due after five years through ten years	167,362	162,061
Due after ten years	12,329	18,591
Amortized Cost	587,854	601,407
Fair Value
Due in one year or less	87,434	51,932
Due in one year or less	313,052	367,377
Due after five years through ten years	160,599	160,374
Due after ten years	11,585	18,192
Total	572,670	597,875
Mortgage-backed securities
Amortized Cost
Amortized Cost	772,654	807,589
Fair Value
Total	$ 738,002	$ 793,988