Investment Securities - Narrative (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Fair value of investments	$ 446,047,000	$ 510,297,000
Unrealized losses on mortgage-backed securities portfolio	4,514,000	9,674,000
Unrealized losses in available for sale and held to maturity portfolios	2,083,000	3,802,000
Carrying value of securities pledged as collateral	475,999,000	572,896,000	$ 637,358,000
Securities sold under repurchase agreements	136,639,000	145,936,000
Gross gains on securities sold	2,681,000	3,389,000	2,770,000
Gross losses from sale of available for sale securities	50,000	0	$ 100,000
Investments in Debt and Equity Securities, Reported at Less than Historical Cost
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Historical cost	452,644,000	523,773,000
Fair value of investments	$ 446,047,000	$ 510,297,000
Percent of the Corporation's available for sale and held to maturity portfolio	28.60%	39.10%
U.S. Government-Sponsored Mortgage-backed Securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on mortgage-backed securities portfolio	$ 3,423,000	$ 3,071,000
Available-for-sale, number of securities in unrealized loss positions | security	63
Unrealized losses in available for sale and held to maturity portfolios	$ 1,404,000	1,527,000
Held-to-maturity, number of securities in unrealized loss positions | security	36
State and Municipal Securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on mortgage-backed securities portfolio	$ 1,091,000	5,564,000
Available-for-sale, number of securities in unrealized loss positions | security	42
Unrealized losses in available for sale and held to maturity portfolios	$ 244,000	1,796,000
Held-to-maturity, number of securities in unrealized loss positions | security	28
U.S. Government-sponsored agency securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on mortgage-backed securities portfolio	$ 0
Unrealized losses in available for sale and held to maturity portfolios	$ 435,000	479,000
Held-to-maturity, number of securities in unrealized loss positions | security	5
Equity securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on mortgage-backed securities portfolio		$ 1,039,000