Pension Plans and Other Postretirement Benefits Financial Assets Accounted for at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Total Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 606,957		$ 619,995
Total Pension Benefits | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	146,683		163,408
Total Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	116,649		115,949
Total Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	255,401		254,560
Total Pension Benefits | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	86,112		80,746
Total Pension Benefits | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,112		5,332
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,232	[5]	3,292	[5]	$ 4,439
Other Postretirement Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,232	[1]	3,292	[3]
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		460,247		481,837
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	143,987		161,075
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	83,839		84,019
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	230,309		231,411
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Quoted Prices in Active Markets for Identical Items (Level 1) | Total Pension Benefits | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,112		5,332
Quoted Prices in Active Markets for Identical Items (Level 1) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Quoted Prices in Active Markets for Identical Items (Level 1) | Other Postretirement Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	[1]	0	[3]
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		60,598		57,412
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	2,696		2,333
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	32,810		31,930
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	25,092		23,149
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Quoted Prices in Active Markets for Similar Items (Level 2) | Total Pension Benefits | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Quoted Prices in Active Markets for Similar Items (Level 2) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	2,232		3,292
Quoted Prices in Active Markets for Similar Items (Level 2) | Other Postretirement Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,232	[1]	3,292	[3]
Unobservable Inputs (Level 3) | Total Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Unobservable Inputs (Level 3) | Total Pension Benefits | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	0		0
Unobservable Inputs (Level 3) | Total Pension Benefits | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	0		0
Unobservable Inputs (Level 3) | Total Pension Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	0		0
Unobservable Inputs (Level 3) | Total Pension Benefits | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Unobservable Inputs (Level 3) | Total Pension Benefits | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Unobservable Inputs (Level 3) | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0		0
Unobservable Inputs (Level 3) | Other Postretirement Benefits | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 0	[1]	$ 0	[3]

[1] Consists primarily of U.S. stock funds that track or are actively managed and measured against the S&P 500 index.

[2] Consists primarily of international equity funds which invest in common stocks and other securities whose value is based on an international equity index or an underlying equity security or basket of equity securities.

[3] Consists primarily of debt obligations issued by governments, corporations, municipalities and other borrowers. Also includes insurance policies.

[4] Consists primarily of funds with holdings in private investment companies. See additional information about the Absolute Return investments below. Holdings in private investment companies are measured at fair value using the net asset value per share as a practical expedient and have not been categorized in the fair value hierarchy as a result of the adoption of new accounting guidance effective January 1, 2016 on a retrospective basis. The fair value amounts of $86.1 million and $80.7 million as of December 31, 2016 and 2015, respectively, are included in this table to permit reconciliation to the reconciliation of plan assets table above. See Note 1, “Summary of Significant Accounting Policies,” for additional information.

[5] See Note 15 “Pension Plans and Other Postretirement Benefits” for further information about fair value measurements of our pension and postretirement plan assets