Employee Benefit Plans and Other Postretirement Benefits (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Retirement age threshold			55 years
Postretirement benefits eligibility, years of service requirement			10 years
Period over which asset gain (losses) are amortized			5 years
Company contributions			$ 18,200	$ 18,000	$ 18,100
Pension and SERPA Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			1,900,000	1,920,000
Projected benefit obligations			$ 1,986,435	$ 2,009,000	$ 2,069,980
Expected return on plan assets			7.50%	7.75%	7.75%
Company contributions			$ 25,000	$ 0
SERPA Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation			38,400	35,800
Projected benefit obligations			$ 52,300	$ 45,000
Pension Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Investment in company common stock - shares			1,273,592	1,273,592
Investment in company common stock - value			$ 74,300	$ 57,800
Pension Plan Assets | Subsequent Event
Defined Benefit Plan Disclosure [Line Items]
Company contributions	$ 25,000
Pension Plan Assets | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets		7.25%
Pension Plan Assets | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocation			63.00%
Pension Plan Assets | Fixed Income Securities and Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocation			37.00%
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligations			$ 346,431	$ 354,739	$ 361,006
Expected return on plan assets			7.50%	7.70%	7.70%
Company contributions			$ 0	$ 0
Postretirement Health Coverage | Scenario, Forecast
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets		7.25%
Postretirement Health Coverage | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocation			69.00%
Postretirement Health Coverage | Fixed Income Securities and Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Target plan asset allocation			31.00%