Investment Securities - Narrative (Details)	12 Months Ended
Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Fair value of investments	$ 932,577,000	$ 446,047,000
Unrealized losses on available for sale securities	14,253,000	4,514,000
Unrealized losses in held to maturity portfolios	4,741,000	2,083,000
Carrying value of securities pledged as collateral	416,155,000	475,999,000	$ 572,896,000
Securities sold under repurchase agreements	116,691,000	136,639,000
Gross gains on securities sold	4,269,000	2,681,000	3,389,000
Gross losses from sale of available for sale securities	0	50,000	$ 0
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	951,571,000	452,644,000
Fair value of investments	$ 932,577,000	$ 446,047,000
Percent of the Corporation's available for sale and held to maturity portfolio	57.10%	28.60%
U.S. Government-Sponsored Mortgage-backed Securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on available for sale securities	$ 8,396,000	$ 3,423,000
Available-for-sale number of securities in unrealized loss positions \| security	105
Unrealized losses in held to maturity portfolios	$ 3,323,000	1,404,000
Held-to-maturity, number of securities in unrealized loss positions \| security	76
State and municipal securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on available for sale securities	$ 5,854,000	1,091,000
Available-for-sale number of securities in unrealized loss positions \| security	221
Unrealized losses in held to maturity portfolios	$ 872,000	244,000
Held-to-maturity, number of securities in unrealized loss positions \| security	41
U.S. Government-sponsored agency securities
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on available for sale securities	$ 3,000
Available-for-sale number of securities in unrealized loss positions \| security	4
Unrealized losses in held to maturity portfolios	$ 545,000	435,000
Held-to-maturity, number of securities in unrealized loss positions \| security	5
Foreign Investment
Schedule of Available for sale Securities and Held to maturity Securities [Line Items]
Unrealized losses on available for sale securities	$ 0
Unrealized losses in held to maturity portfolios	$ 1,000
Held-to-maturity, number of securities in unrealized loss positions \| security	1

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Schedule of Available for sale Securities and Held to maturity Securities [Line Items]

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Securities, Continuous Unrealized Loss Position, Fair Value

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Disclosure regarding factors used to determine that the impairment of securities categorized as held-to-maturity where cost exceeds fair value is not an other than temporary impairment (OTTI). This item contains disclosure of the number of investment positions in the held-to-maturity investments determined to be temporarily impaired.

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Amount of asset associated with funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price.

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