Financial Instruments (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 EUR (€)	Jun. 30, 2017 USD ($)
Gain (loss) from the change in fair value of financial instruments	$ 1,000,000	$ (3,000,000)	$ 1,400,000	$ (2,700,000)
Notional amount	345,300,000		345,300,000		$ 238,500,000
Interest Rate Swaps [Member] | Cash Flow Hedging [Member]
Notional amount	319,200,000		319,200,000
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	1,200,000		1,200,000
Cross Currency Swaps [Member] | Cash Flow Hedging [Member]
Notional amount	230,000,000		230,000,000
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	5,500,000		5,500,000
Cross Currency Swaps [Member] | Net Investment Hedging [Member]
Notional amount	$ 117,500,000		$ 117,500,000
Secured Debt [Member] | Interest Rate Swaps [Member] | Cash Flow Hedging [Member]
Debt amount issued						€ 90,000,000	$ 150,000,000