Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net unrealized holding gains in available for sale securities, excluded from deferred tax assets (liabilities) calculation	$ 4,300,000	$ 5,100,000
Employee benefit plan deferred tax assets, excluded from deferred tax asset calculation	12,600,000	18,600,000
Federal tax reform remeasurement of AOCI deferred tax asset	9,958,000	0	$ 0
Deferred tax assets, valuation allowance	0	0
Unrecognized tax benefits	0	$ 0
Re-measurement of net deferred tax asset resulting in additional income tax expense	$ 14,900,000