Long-Term Debt, Net of Current (Narrative) (Details)	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2021	Sep. 30, 2020	Apr. 30, 2020 USD ($)	Sep. 30, 2019	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2016 USD ($)	Apr. 30, 2014 USD ($)	Apr. 30, 1998 USD ($)
Debt Instrument [Line Items]
Accrued interest expense		$ 1,000,000		$ 1,000,000										$ 800,000
Debt Issuance Costs, Gross	$ 6,300,000	14,300,000	$ 6,300,000	14,300,000	$ 6,300,000
Debt Issuance Costs, Net	300,000	300,000	300,000	300,000	300,000
Amortization of Debt Issuance Costs	6,000,000	14,000,000	6,000,000	14,000,000	6,000,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net		13,800,000		13,800,000
Debt Instrument, Unamortized Discount		12,000,000		12,000,000
Write off of Deferred Debt Issuance Cost				1,800,000	10,200,000
Market value adjustments on interest rate swap, net of tax		41,000	365,000	1,621,000	(2,673,000)
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Swaps notional amount		$ 275,000,000		$ 275,000,000											$ 500,000,000	$ 500,000,000
Swaps fixed interest rate		1.83%		1.83%											1.03%	1.57%
Asset for interest rate swap agreements														5,400,000
Liability for swap														$ 2,300,000
Interest Rate Swap [Member] | Forecast [Member]
Debt Instrument [Line Items]
Derivative, Notional Amount, Increase Swap B										$ 200,000,000
Quarterly amortization of interest rate swap notional amount								$ 25,000,000			$ 25,000,000		$ 12,500,000
Increase in notional amount										$ 100,000,000
Remaining notional amount of swaps								$ 275,000,000
Swap [Member]
Debt Instrument [Line Items]
Market value adjustments on interest rate swap, net of tax		$ 100,000
Deferred taxes on interest rate swaps		100,000	$ 200,000	$ 1,000,000	$ (1,700,000)
Minimum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Commitment Fee Percentage				0.25%
Maximum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Commitment Fee Percentage				0.40%
Adjusted LIBO [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Adjusted LIBO [Member] | Minimum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.25%
Adjusted LIBO [Member] | Maximum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				2.00%
Adjusted LIBO Plus [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.00%
Alternate Base Rate [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.00%
Alternate Base Rate [Member] | Minimum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				0.25%
Alternate Base Rate [Member] | Maximum [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				1.00%
Federal Funds Effective Rate Plus [Member] | Credit Agreement [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate				0.50%
Term Facility [Member]
Debt Instrument [Line Items]
Credit agreement, potential increase to term loan and line of credit		100,000,000		$ 100,000,000
Term Facility [Member] | Term Loan A-1 Due August 2022 [Member]
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity		$ 1,800,000,000		$ 1,800,000,000
Term Facility [Member] | Term Loan Due August 2022 [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		3.24%		3.24%
Term Facility [Member] | Term Loan Due April 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate														2.31%
Term Facility [Member] | Debt Instrument, Redemption, Period One [Member] | Term Loan Due April 2020 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Periodic Payment	22,500,000
Term Facility [Member] | Debt Instrument, Redemption, Period Two [Member] | Term Loan Due April 2020 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Periodic Payment	$ 45,000,000
Line of credit [Member] | Forecast [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Restrictive Covenants, Leverage Ratio						3.50	3.75		4.00			4.25
Line of credit [Member] | Revolving Facility [Member]
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity		$ 700,000,000		$ 700,000,000
Multicurrency revolving sub-facility		100,000,000		100,000,000
Revolving credit facility, letter of credit sub-facility		50,000,000		50,000,000
Revolving line of credit, remaining borrowing capacity		$ 700,000,000		$ 700,000,000
Long-term Debt, Weighted Average Interest Rate														2.31%
Line of credit [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Restrictive Covenants, Interest Coverage Ratio		3.50		3.50
Line of credit [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Restrictive Covenants, Leverage Ratio		4.50		4.50
Senior notes [Member] | 7.55% senior debentures due April 2028 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount																	$ 100,000,000
Debt instrument, stated interest rate percentage		7.55%		7.55%													7.55%