Pensions and Other Postretirement Benefit Plans (Schedule of Fair Value of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Investment of plan assets	$ 16,200	$ 7,800
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Investment of plan assets
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	205,586	180,672	171,387
Investment of plan assets	16,242	7,794
Pension Plans [Member] | Total investments in the fair value hierarchy [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87,358	80,397
Pension Plans [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,588	3,710
Pension Plans [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81,363	74,449
Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,407	2,238	2,403
Common Stocks and Equity Funds [Member] | Other postretirement benefits [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	335	309
Common Stocks and Equity Funds [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	335	309
Investment of plan assets	37,768	35,510
Common Stocks and Equity Funds [Member] | Pension Plans [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Common Stocks and Equity Funds [Member] | Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Debt Securities [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81,363	74,449
Debt Securities [Member] | Pension Plans [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Debt Securities [Member] | Pension Plans [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81,363	74,449
Debt Securities [Member] | Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Fixed income funds [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment of plan assets	75,881	59,662
Insurance Contracts [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,407	2,238
Insurance Contracts [Member] | Pension Plans [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Insurance Contracts [Member] | Pension Plans [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Insurance Contracts [Member] | Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,407	2,238	$ 2,403
Cash and Short-Term Investments [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,253	3,401
Cash and Short-Term Investments [Member] | Pension Plans [Member] | Quoted Prices in Active Markets (Level 1) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,253	3,401
Cash and Short-Term Investments [Member] | Pension Plans [Member] | Significant Other Observable Inputs (Level 2) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Cash and Short-Term Investments [Member] | Pension Plans [Member] | Significant Unobservable Inputs (Level 3) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets
Limited Partnerships [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment of plan assets	4,579	5,065
Hedge Funds [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Investment of plan assets		$ 38