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Securities - Contractual Maturities of Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Debt Securities, Available-for-Sale, Amortized Cost	$ 693,008
Fair Value
Debt Securities, Available-for-Sale, Fair Value	682,916	$ 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	33,878
Due after one year through five years	152,588
Due after five years through ten years	73,099
Due after ten years	28,692
Debt Securities Available-for-sale, Maturity, Amortized Cost	288,257
Fair Value
Due in one year or less	33,888
Due after one year through five years	152,374
Due after five years through ten years	72,961
Due after ten years	29,381
Debt Securities Available-for-sale, Maturity, Fair Value	288,604
Collateralized mortgage obligations of U.S. government corporations and agencies
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	121,631
Debt Securities, Available-for-Sale, Amortized Cost	121,631	109,916
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	118,742
Debt Securities, Available-for-Sale, Fair Value	118,742	108,688
Residential mortgage-backed securities of U.S. government corporations and agencies
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	28,662
Debt Securities, Available-for-Sale, Amortized Cost	28,662	32,388
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	28,442
Debt Securities, Available-for-Sale, Fair Value	28,442	32,854
Commercial mortgage-backed securities of U.S. government corporations and agencies
Amortized Cost
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Amortized Cost	254,458
Debt Securities, Available-for-Sale, Amortized Cost	254,458	244,018	[1]
Fair Value
Debt Securities, Available-for-sale, Maturity, without Single Maturity Date, Fair Value	247,128
Debt Securities, Available-for-Sale, Fair Value	$ 247,128	$ 242,221	[1]
[1] Includes a $5.9 million security purchase that was pending settlement as of December 31, 2017.

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