Debt (Narrative) (Details) € in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2015 EUR (€)	Dec. 31, 2015 USD ($)	Sep. 30, 2014 USD ($)	Jul. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Line of Credit Facility [Line Items]
Outstanding Credit Facility balance		$ 655,000,000	$ 611,000,000
Credit Facility borrowings hedged		$ 80,000,000
Consolidated Leverage Ratio Under Credit Facility & Note Payable, maximum		3.50%
Interest paid		$ 37,600,000	31,800,000	$ 27,200,000
Maximum [Member]
Line of Credit Facility [Line Items]
Commitment fees		0.25%
Minimum [Member]
Line of Credit Facility [Line Items]
Commitment fees		0.075%
Senior Notes [Member]
Line of Credit Facility [Line Items]
Consolidated Leverage Ratio Under Credit Facility & Note Payable, maximum		3.50%
Debt instrument, face amount							$ 75,000,000	$ 125,000,000	$ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage							3.72%
Series A Senior Note [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 75,000,000							$ 50,000,000	$ 75,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.25%							3.32%	3.94%
Debt Instrument, Term	7 years
Series B Senior Note [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 75,000,000							$ 75,000,000	$ 75,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.72%							3.76%	4.04%
Debt Instrument, Term	12 years
Series C Senior Note [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount | €					€ 88.9
Debt Instrument, Interest Rate, Stated Percentage					1.785%	1.785%
Shelf Notes [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount						$ 75,000,000
Debt Instrument, Term				12 years
Agreement Term				3 years
Outdated Unsecured Short Term Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Borrowing capacity						$ 700,000,000
Amended And Restated Unsecured Short Term Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Borrowing capacity		$ 850,000,000
Credit Facility maturity date		Dec. 04, 2020
Outstanding Credit Facility balance		$ 655,000,000	$ 611,000,000
Credit Facility weighted average interest rates on outstanding balance		2.81%	1.95%
Reduction of Credit Facility Availability		$ 1,000,000	$ 1,000,000
Debt Instrument, Term		5 years
Minimum Credit Spread On LIBOR Or CDOR Borrowings [Member]
Line of Credit Facility [Line Items]
Credit spread during period		0.875%
Maximum Credit Spread On LIBOR Or CDOR Borrowings [Member]
Line of Credit Facility [Line Items]
Credit spread during period		1.375%
Maximum Credit Spread On Prime Rate Borrowings [Member]
Line of Credit Facility [Line Items]
Credit spread during period		0.375%
Uncommitted Shelf Facility [Member]
Line of Credit Facility [Line Items]
Debt instrument, face amount	$ 50,000,000
Agreement Term				3 years
Uncommitted Shelf Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Term	15 years