Long-Term Debt, Net of Current (Narrative) (Details)	1 Months Ended				12 Months Ended
	Nov. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jul. 31, 2016 USD ($)	Apr. 30, 2010	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	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	May 31, 2011 USD ($)	Apr. 30, 1998 USD ($)
Debt Instrument [Line Items]
Gross					$ 1,619,279,000	$ 1,364,008,000
Accrued interest expense					800,000	3,600,000
Premium on debt extinguishment payment					16,271,000	0	$ 0
Debt Issuance Costs					6,300,000	6,500,000
Debt Issuance Costs Expensed					300,000	400,000
Debt issuance costs capitalized					6,000,000	6,100,000
Unamortized costs related to previously recorded debt issuance costs					14,000,000
Unamortized debt issuance costs written off					10,200,000	1,600,000
Market value adjustments on interest rate swap, net of tax					4,618,000	(364,000)	(2,408,000)
Interest Rate Swap [Member]
Debt Instrument [Line Items]
Swaps, Notional Amount					$ 500,000,000		$ 500,000,000
Swaps, Fixed Interest Rate					1.03%		1.57%
Liability for interest rate swap agreements					$ 2,300,000	4,400,000
Asset for interest rate swap agreements					5,400,000
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
Derivative, Notional Amount, Increase										$ 100,000,000
Swap [Member]
Debt Instrument [Line Items]
Deferred taxes on interest rate swaps					$ 2,900,000	(200,000)	$ (1,500,000)
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%
Term Loan Facility [Member]
Debt Instrument [Line Items]
Credit Agreement, Potential Increase to Term Loan and Line of Credit			$ 225,000,000
Line of Credit Facility, Amended and Increased			525,000,000
Term Loan Facility [Member] | Term Loan Due April 2020 [Member]
Debt Instrument [Line Items]
Gross					$ 1,298,125,000	828,750,000
Term Facility, Maximum Borrowing Capacity			1,300,000,000
Term Loan Facility [Member] | Term Loan Due April 2020 [Member] | Debt Instrument, Redemption, Period One [Member]
Debt Instrument [Line Items]
Term Facility, Periodic Payment		$ 17,200,000
Term Loan Facility [Member] | Term Loan Due April 2020 [Member] | Debt Instrument, Redemption, Period Two [Member]
Debt Instrument [Line Items]
Term Facility, Periodic Payment		34,400,000
Term Loan Facility [Member] | Term Loan Due April 2020 [Member] | Debt Instrument, Repayments, 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 Restrictive Covenants, Interest Coverage Ratio									3.50			3.75		4.25
Line of Credit [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Restrictive Covenants, Interest Coverage Ratio					3.00
Line of Credit [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Restrictive Covenants, Interest Coverage Ratio					4.50
Line of Credit [Member] | Revolving facility [Member]
Debt Instrument [Line Items]
Gross					$ 302,000,000	75,000,000
Term Facility, Maximum Borrowing Capacity			550,000,000
Multicurrency Revolving Sub-Facility			100,000,000
Line of Credit Facility, Letter Of Credit Sub-Facility			50,000,000
Revolving Line of Credit, Remaining Borrowing Capacity					248,000,000
Senior Notes [Member] | 7.25% senior notes due June 2021 [Member]
Debt Instrument [Line Items]
Gross					$ 0	393,000,000
Debt instrument, stated rate percentage					7.25%										7.25%
Debt instrument,face amount															$ 400,000,000
Repayments of Debt			$ 411,000,000
Premium on debt extinguishment payment					$ (14,246,000.0000)
Senior Notes [Member] | 7.55% senior debentures due April 2028 [Member]
Debt Instrument [Line Items]
Gross					$ 14,645,000	$ 59,645,000
Debt instrument, stated rate percentage					7.55%											7.55%
Debt instrument,face amount																$ 100,000,000
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%
Bank Loans [Member] | Term Loan Due March 2019 [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs written off					$ 10,200,000