Employee Benefit Plans - 401K Contribution and Supplemental Compensation agreement (Details) - USD ($)		1 Months Ended	12 Months Ended
	Aug. 18, 2015	Sep. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 27, 2015
Employee Benefit Plans
Base contribution as a percentage of employee gross pay (as a percent)			2.00%
Employer maximum match of an employee's contributions of first 4% of eligible compensation (as a percent)			100.00%
Percentage of eligible compensation, matched 100% by employer			4.00%
Company's matching contributions under certain 401(k) savings plans			$ 5,000,000	$ 5,400,000	$ 4,300,000
Charges for pension plans			$ 4,100,000	$ 4,500,000	$ 4,900,000
Deferred tax assets on cumulative actuarial losses						$ 23,000,000
SERP
Employee Benefit Plans
Settlement charge		$ 43,200,000
Timothy P. Horne
Components of net periodic benefit cost
Consulting service payment (as a percent)					50.00%
Period of annual base salary	3 years
Annual payment for consulting services	$ 400,000
Payments for consulting service as per compensation agreement	598,562
Lump sum buyout of consulting services lifetime payment obligation	$ 6,000,000
Payment for lump-sum buyout of lifetime consulting service payment obligation		$ 6,000,000
Pre-tax charge for lump-sum buyout of lifetime payment obligation					$ 5,000,000