Contingent Liabilities (Details) - USD ($)	1 Months Ended			6 Months Ended
	Jul. 29, 2016	Dec. 31, 2014	Sep. 30, 2013	Jul. 01, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Amount paid related to Revised Assessment	$ 5,000,000
Letters of credit				$ 59,800,000	$ 56,000,000
Threatened Litigation
Loss Contingencies [Line Items]
Demand for payment		$ 34,000,000	$ 26,000,000
Decline in contributions, percent (more than)			70.00%
Payments related to Initial Assessment				17,100,000
Partial pension withdrawal arbitration liability				4,900,000
Minimum | Threatened Litigation
Loss Contingencies [Line Items]
Estimate of possible loss				0
Maximum | Threatened Litigation
Loss Contingencies [Line Items]
Estimate of possible loss				10,000,000
Letter of Credit
Loss Contingencies [Line Items]
Collateral, letters of credit				$ 68,000,000	$ 63,000,000