Long-Term Debt (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2025	Aug. 31, 2022	Mar. 31, 2021	Jan. 01, 2021	Dec. 31, 2020	Dec. 31, 2017	Aug. 31, 2017	Apr. 30, 1998
Debt Instrument [Line Items]
Accrued interest expense	$ 1,500,000		$ 1,500,000							$ 1,000,000
Credit agreement, potential increase to term loan and line of credit	100,000,000		100,000,000
Debt issuance costs											$ 14,300,000
Debt issuance costs capitalized											14,000,000
Unamortized costs											$ 12,000,000
Debt issuance costs expensed	1,400,000	$ 1,400,000	4,103,000	$ 4,263,000
Market value adjustments on interest rate swaps, net of tax	2,532,000	41,000	10,770,000	1,621,000
Interest Rate Swap
Debt Instrument [Line Items]
Remaining notional balance	$ 1,300,000,000		$ 1,300,000,000
Derivative, average fixed interest rate	1.80%		1.80%
Asset for Swaps										12,000,000
Interest Rate Swap | Minimum
Debt Instrument [Line Items]
Derivative, average fixed interest rate	1.03%		1.03%
Interest Rate Swap | Maximum
Debt Instrument [Line Items]
Derivative, average fixed interest rate	2.98%		2.98%
Interest Rate Swap | Recurring
Debt Instrument [Line Items]
Asset for Swaps	$ 26,856,000		$ 26,856,000							11,985,000
Interest Rate Swap | Recurring | Level 2
Debt Instrument [Line Items]
Asset for Swaps	26,856,000		26,856,000							$ 11,985,000
Interest Rate Swap | Forecast
Debt Instrument [Line Items]
Remaining notional balance					$ 400,000,000	$ 1,000,000,000	$ 1,300,000,000	$ 1,200,000,000	$ 1,500,000,000
Interest Rate Swap | Current Assets | Recurring | Level 2
Debt Instrument [Line Items]
Asset for Swaps	1,100,000		1,100,000
Swap
Debt Instrument [Line Items]
Market value adjustments on interest rate swaps, net of tax	2,500,000	100,000
Deferred taxes on interest rate swaps	800,000	$ 100,000	3,600,000	$ 1,000,000
Revolving line of credit borrowings due August 2022, weighted-average interest rate of 3.84% as of September 30, 2018 | Revolving line of credit
Debt Instrument [Line Items]
Multicurrency revolving sub-facility	100,000,000		100,000,000
Revolving line of credit borrowings due August 2022, weighted-average interest rate of 3.84% as of September 30, 2018 | Letter of Credit
Debt Instrument [Line Items]
Revolving credit facility, letter of credit sub-facility	50,000,000		50,000,000
Term loan facility | Term Loan A-1 Due August 2022
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity	1,800,000,000		$ 1,800,000,000
Term of loan facility			5 years
Line of credit | Revolving line of credit
Debt Instrument [Line Items]
Term facility, maximum borrowing capacity	700,000,000		$ 700,000,000
Revolving line of credit, remaining borrowing capacity	$ 580,000,000		$ 580,000,000
Senior notes | 7.55% senior debentures due April 2028
Debt Instrument [Line Items]
Debt instrument, face amount												$ 100,000,000