Securities - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Amortized Cost	$ 690,251
Debt Securities, Fair Value	682,454	$ 693,147
Obligations of the U.S. Treasury, U.S. government corporations and agencies, and obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	43,472
Due after one year through five years, Amortized Cost	148,616
Due after five years through ten years, Amortized Cost	78,565
Due after ten years, Amortized Cost	28,792
Debt Securities, Amortized Cost	299,445
Due in one year or less, Fair Value	43,372
Due after one year through five years, Fair Value	148,787
Due after five years through ten years, Fair Value	78,767
Due after ten years, Fair Value	29,465
Debt Securities, Fair Value	300,391
Collateralized mortgage obligations of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Amortized Cost	105,063	109,916
Debt Securities, Fair Value	102,582	108,688
Residential mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Amortized Cost	30,700	32,388
Debt Securities, Fair Value	30,683	32,854
Commercial mortgage-backed securities of U.S. government corporations and agencies
Schedule of Available-for-sale Securities [Line Items]
Debt Securities, Amortized Cost	255,043	244,018	[1]
Debt Securities, Fair Value	$ 248,798	$ 242,221	[1]

[1] Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.