Securities - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized Cost
Debt Securities, Available-for-Sale, Amortized Cost	$ 690,834
Fair Value
Debt Securities, Available-for-Sale, Fair Value	677,221	$ 693,147
Obligations of the U.S. Treasury, U.S. government corporations and agencies, and obligations of states and political subdivisions
Amortized Cost
Due in one year or less	20,871
Due after one year through five years	147,573
Due after five years through ten years	72,590
Due after ten years	28,590
Debt Securities Available-for-sale, Maturity, Amortized Cost	269,624
Fair Value
Due in one year or less	20,878
Due after one year through five years	146,653
Due after five years through ten years	71,938
Due after ten years	29,046
Debt Securities Available-for-sale, Maturity, Fair Value	268,515
Collateralized mortgage obligations of U.S. government corporations and agencies
Amortized Cost
Debt Securities Available-for-Sale With Maturities	141,283
Debt Securities, Available-for-Sale, Amortized Cost	141,283	109,916
Fair Value
Debt Securities Available-for-Sale With Maturities	137,660
Debt Securities, Available-for-Sale, Fair Value	137,660	108,688
Residential mortgage-backed securities of U.S. government corporations and agencies
Amortized Cost
Debt Securities Available-for-Sale With Maturities	26,837
Debt Securities, Available-for-Sale, Amortized Cost	26,837	32,388
Fair Value
Debt Securities Available-for-Sale With Maturities	26,450
Debt Securities, Available-for-Sale, Fair Value	26,450	32,854
Commercial mortgage-backed securities of U.S. government corporations and agencies
Amortized Cost
Debt Securities Available-for-Sale With Maturities	253,090
Debt Securities, Available-for-Sale, Amortized Cost	253,090	244,018	[1]
Fair Value
Debt Securities Available-for-Sale With Maturities	244,596
Debt Securities, Available-for-Sale, Fair Value	$ 244,596	$ 242,221	[1]

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