Long-Term Debt, Net of Current (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Apr. 30, 2020	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Aug. 31, 2016	Jul. 31, 2016	Apr. 30, 2014	Apr. 30, 1998
Debt Instrument [Line Items]
Accrued interest expense	$ 600,000		$ 600,000						$ 800,000
Market value adjustments on interest rate swap, net of tax	50,000	$ (439,000)	1,580,000	$ (3,038,000)
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]
Deferred taxes on interest rate swaps	$ 100,000	$ (300,000)	$ 1,000,000	$ (1,900,000)
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
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	$ 178,000,000		$ 178,000,000
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%