Long-Term Debt (Narrative) (Details)	1 Months Ended			12 Months Ended
	Nov. 30, 2016 USD ($)	Jul. 31, 2016 USD ($)	Apr. 30, 2010	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 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	Aug. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Apr. 30, 2014 USD ($)	May 31, 2011 USD ($)	Apr. 30, 1998 USD ($)
Debt Instrument [Line Items]
Accrued interest expense				$ 1,000,000	$ 800,000
Debt issuance costs					6,300,000									$ 14,300,000
Debt Issuance Costs Expensed				2,100,000	300,000
Debt issuance costs capitalized					6,000,000									$ 14,000,000
Unamortized costs related to previously recorded debt issuance costs				13,800,000
Unamortized debt issuance costs written off				1,800,000	10,200,000
Debt Instrument, Unamortized Discount				12,000,000
Market value adjustments on interest rate swaps, net of tax				5,481,000	4,618,000	$ (364,000)
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Swaps, Notional Amount				$ 275,000,000											$ 500,000,000	$ 500,000,000
Swaps, Fixed Interest Rate				1.83%											1.03%	1.57%
Quarterly Amortization of Interest Rate Swap Notional Amount				$ 12,500,000
Liability for interest rate swap agreements					2,300,000
Asset for interest rate swap agreements				12,000,000	5,400,000
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
Remaining Balance Of Interest Rate Swap Notional Amount									$ 275,000,000
Derivative, Notional Amount, Increase											$ 100,000,000
Swap [Member]
Debt Instrument [Line Items]
Deferred taxes on interest rate swaps				3,400,000	2,900,000	$ (200,000)
Line of Credit due August 2022 [Member] | Revolving facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 700,000,000
Term of loan				5 years
Multicurrency revolving sub-facility				$ 100,000,000
Letter of credit sub-facility				$ 50,000,000
Credit Agreement [Member] | Minimum [Member]
Debt Instrument [Line Items]
Revolving credit facility, commitment fee percentage				0.25%
Credit Agreement [Member] | Maximum [Member]
Debt Instrument [Line Items]
Revolving credit facility, commitment fee percentage				0.40%
Credit Agreement [Member] | Federal Funds Effective Rate Plus [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				0.50%
Credit Agreement [Member] | Adjusted LIBO Plus [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				1.00%
Credit Agreement [Member] | Alternate Base Rate [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				1.00%
Credit Agreement [Member] | Alternate Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				0.25%
Credit Agreement [Member] | Alternate Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				1.00%
Credit Agreement [Member] | Adjusted LIBO [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				2.00%
Credit Agreement [Member] | Adjusted LIBO [Member] | Minimum [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				1.25%
Credit Agreement [Member] | Adjusted LIBO [Member] | Maximum [Member]
Debt Instrument [Line Items]
Basis of spread on variable rate on debt				2.00%
Term Loan Facility [Member]
Debt Instrument [Line Items]
Potential increase to term loan and line of credit				$ 100,000,000
Amount notes amended and increased		$ 525,000,000
Term Loan Facility [Member] | Term Loan A-1 Due August 2022 [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 1,800,000,000
Term of loan				5 years
Term Loan Facility [Member] | Term Loan Due April 2020 [Member] | Debt Instrument, Redemption, Period One [Member]
Debt Instrument [Line Items]
Term Facility, periodic payment				$ 22,500,000
Term Loan Facility [Member] | Term Loan Due April 2020 [Member] | Debt Instrument, Redemption, Period Two [Member]
Debt Instrument [Line Items]
Term Facility, periodic payment				$ 45,000,000
Line of Credit [Member] | Forecast [Member]
Debt Instrument [Line Items]
Debt covenants, leverage ratio							3.50	3.75		4.00			4.25
Line of Credit [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt covenants, interest coverage ratio				3.50
Line of Credit [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt covenants, leverage ratio				4.50
Line of Credit [Member] | Revolving facility [Member]
Debt Instrument [Line Items]
Remaining borrowing capacity				$ 700,000,000
Senior Notes [Member] | 7.25% senior notes due June 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount																	$ 400,000,000
Debt instrument, stated rate percentage																	7.25%
Repayments of debt		$ 411,000,000
Premium on debt extinguishment payment					14,200,000
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 rate percentage				7.55%														7.55%
Repayments of debt	$ 45,000,000
Premium on debt extinguishment payment					$ 2,000,000
Senior Notes [Member] | 7.55% senior debentures due April 2028 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Valid Consents Tendered, Percentage Of Outstanding Debentures			50.00%