Employee Benefit Plans - Narrative (Details)	1 Months Ended	10 Months Ended	12 Months Ended
	Nov. 30, 2017	Oct. 31, 2017	Dec. 31, 2017 USD ($) Retiree employee shares	Dec. 31, 2016 USD ($) Retiree shares	Dec. 31, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of retirees and dependents | Retiree			340	310
Subsidy limit percentage of expected annual fully insured premium over age threshold			60.00%
Subsidy limit percentage of expected annual fully insured premium under age threshold			$ 648,000
Annual subsidy limit percentage increases for fully insured premium over age threshold			3.50%
Estimated contributions in 2016			$ 1,700,000
Liabilities, including the Company's common stock			29,145,000	$ 24,169,000
Defined contribution cost (credit) recognized			$ 2,600,000	1,800,000	$ (6,400,000)
Savings Investment Plan
Defined Benefit Plan Disclosure [Line Items]
Employer matching percent			6.00%
Maximum contribution, percent of employee salary	10.00%	9.00%
Plan contributions charged to expense			$ 6,500,000	6,500,000	7,100,000
Deferred compensation plan
Defined Benefit Plan Disclosure [Line Items]
Market value of the trust assets, excluding the Company's common stock			15,000,000	12,600,000
Liabilities, including the Company's common stock			$ 29,100,000	$ 24,200,000
Number of common stock deferred into the rabbi trust (in shares) | shares			495,774	495,774
Company's common stock held in the rabbi trust			$ 5,100,000	$ 5,100,000
Company contributions to deferred compensation plan			$ 1,000,000	$ 600,000	$ 1,000,000
West Virginia, Virginia, and Ohio
Defined Benefit Plan Disclosure [Line Items]
Employees terminated | employee			100