Pension and Other Postretirement Benefit Plans (Narrative) (Details) - USD ($)			12 Months Ended
	Aug. 31, 2015	May 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Time horizon			20 years
Investment in company stock			$ 0	$ 0
Plan assets			$ 679,245,000	632,434,000
United States Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)			26.00%
Non U.S. Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)			16.50%
Global Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)			7.50%
Fixed Income Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation (as a percent)			50.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Modification to plan, reduction to pension liability				(26,352,000)
Increase (decrease) to pension liability, plan amendment	$ 17,793,000		$ 0	0
Settlement expense			(10,153,000)	(22,196,000)	$ (19,053,000)
Plan assets			679,245,000	632,434,000	669,039,000
Expected benefit payments in 2018			44,778,000
Pension Benefits | Nonqualified Plan
Defined Benefit Plan Disclosure [Line Items]
Expected benefit payments in 2018			1,785,000
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Increase (decrease) to pension liability, plan amendment		$ (43,598,000)	0	(28,164,000)
Remeasurement of plan liability		1,070,000
Settlement expense			0	0	8,932,000
Income from amortization of prior service credit		$ 235,541,000
Plan assets			0	$ 0	$ 0
Expected benefit payments in 2018			$ 37,464,000