Long-Term Debt, Net of Current (Narrative) (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Apr. 30, 2010	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Apr. 30, 2020 USD ($)	Jun. 30, 2019	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018	Dec. 31, 2017 USD ($)	Jun. 30, 2017	Jul. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Apr. 30, 1998 USD ($)
Debt Instrument [Line Items]
Accrued interest expense	$ 2,400,000		$ 2,400,000		$ 2,400,000										$ 3,600,000
Premium on debt extinguishment payment					(14,246,000)	$ 0
Debt issuance costs	6,300,000		6,300,000	$ 6,500,000	6,300,000	6,500,000
Debt issuance costs expensed	300,000		300,000	400,000	300,000	400,000
Debt issuance costs capitalized	6,000,000		6,000,000	6,100,000	6,000,000	6,100,000
Unamortized costs related to previously recorded debt issuance costs	14,000,000		14,000,000		14,000,000
Unamortized debt issuance costs written off			10,200,000		10,200,000	1,600,000
Market value adjustments on interest rate swap, net of tax			365,000	(2,043,000)	(2,673,000)	(3,110,000)
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Swaps notional amount	$ 500,000,000		$ 500,000,000		$ 500,000,000											$ 500,000,000
Swaps fixed interest rate	1.03%		1.03%		1.03%											1.57%
Liability for interest rate swap agreements	$ 8,700,000		$ 8,700,000		$ 8,700,000										$ 4,400,000
Interest Rate Swap [Member] | Forecast [Member]
Debt Instrument [Line Items]
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			$ 250,000,000
Swap [Member]
Debt Instrument [Line Items]
Deferred taxes on interest rate swaps			200,000	$ (1,300,000)	$ (1,700,000)	$ (1,900,000)
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] | Alternate Base Rate [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					0.75%
Credit Agreement [Member] | Alternate Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					0.25%
Credit Agreement [Member] | Alternate Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					1.00%
Credit Agreement [Member] | Adjusted LIBO [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					1.75%
Credit Agreement [Member] | Adjusted LIBO [Member] | Minimum [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					1.25%
Credit Agreement [Member] | Adjusted LIBO [Member] | Maximum [Member]
Debt Instrument [Line Items]
Credit agreement, basis spread on variable rate					2.00%
Senior notes [Member] | 7.25% senior notes due June 2021 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount	$ 400,000,000		$ 400,000,000		$ 400,000,000
Debt instrument, stated interest rate percentage	7.25%		7.25%		7.25%
Premium on debt extinguishment payment			$ (14,200,000)		$ (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 interest rate percentage	7.55%		7.55%		7.55%												7.55%
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%
Term Facility [Member]
Debt Instrument [Line Items]
Credit agreement, potential increase to term loan and line of credit														$ 225,000,000
Term Facility [Member] | Term Loan Due April 2020 [Member]
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity														1,300,000,000
Term Facility [Member] | Term Loan Due April 2020 [Member] | Term loan facility, repayment period one [Member]
Debt Instrument [Line Items]
Term facility, periodic payment	$ 17,200,000
Term Facility [Member] | Term Loan Due April 2020 [Member] | Term loan facility, repayment period two [Member]
Debt Instrument [Line Items]
Term facility, periodic payment	34,400,000
Term Facility [Member] | Term Loan Due April 2020 [Member] | Term loan facility, repayment period three [Member]
Debt Instrument [Line Items]
Term facility, periodic payment	$ 51,600,000
Line of credit [Member] | Forecast [Member]
Debt Instrument [Line Items]
Debt instrument, debt covenants, leverage ratio								3.5			3.75		4.25
Line of credit [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, debt covenants, leverage ratio	3.00		3.00		3.00
Line of credit [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument, debt covenants, leverage ratio	4.50		4.50		4.50
Line of credit [Member] | Revolving Facility [Member]
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity														550,000,000
Multicurrency revolving sub-facility														100,000,000
Revolving credit facility, letter of credit sub-facility														$ 50,000,000
Revolving line of credit, remaining borrowing capacity	$ 295,000,000		$ 295,000,000		$ 295,000,000