Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amortized Cost
Amortized Cost	$ 1,363,902	$ 1,408,996
Fair Value
Total available-for-sale debt securities	1,332,658	1,391,862
Debt securities
Amortized Cost
Due in one year or less	78,160	51,909
Due after one year through five years	355,499	368,846
Due after five years through ten years	139,560	162,061
Due after ten years	9,201	18,591
Amortized Cost	582,420	601,407
Fair Value
Due in one year or less	77,930	51,932
Due after one year through five years	350,470	367,377
Due after five years through ten years	136,734	160,374
Due after ten years	8,668	18,191
Total available-for-sale debt securities	573,802	597,874
Mortgage-backed securities
Amortized Cost
Amortized Cost	781,482	807,589
Fair Value
Total available-for-sale debt securities	$ 758,856	$ 793,988