Pension Plans and Other Postretirement Benefits Financial Assets Accounted for at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 659,874	$ 606,957
Pension Plans | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	163,160	146,683
Pension Plans | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	130,935	116,649
Pension Plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	269,365	255,401
Pension Plans | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	96,414	86,112
Pension Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,112
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	834	2,232	$ 3,292
Other Postretirement Benefits | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	834	2,232
Quoted Prices in Active Markets for Identical Items (Level 1) | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	493,848	460,247
Quoted Prices in Active Markets for Identical Items (Level 1) | Pension Plans | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160,976	143,987
Quoted Prices in Active Markets for Identical Items (Level 1) | Pension Plans | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101,366	83,839
Quoted Prices in Active Markets for Identical Items (Level 1) | Pension Plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	231,506	230,309
Quoted Prices in Active Markets for Identical Items (Level 1) | Pension Plans | 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) | Pension Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		2,112
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	0
Quoted Prices in Active Markets for Similar Items (Level 2) | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,612	60,598
Quoted Prices in Active Markets for Similar Items (Level 2) | Pension Plans | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,184	2,696
Quoted Prices in Active Markets for Similar Items (Level 2) | Pension Plans | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	29,569	32,810
Quoted Prices in Active Markets for Similar Items (Level 2) | Pension Plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37,859	25,092
Quoted Prices in Active Markets for Similar Items (Level 2) | Pension Plans | 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) | Pension Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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	834	2,232
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	834	2,232
Unobservable Inputs (Level 3) | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Unobservable Inputs (Level 3) | Pension Plans | Domestic Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Unobservable Inputs (Level 3) | Pension Plans | International Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Unobservable Inputs (Level 3) | Pension Plans | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Unobservable Inputs (Level 3) | Pension Plans | Absolute Return Measured At Net Asset Value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Unobservable Inputs (Level 3) | Pension Plans | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		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	$ 0