Debt (Narrative) (Details) € in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2016 EUR (€)	Dec. 31, 2016 USD ($)	Sep. 30, 2014 USD ($)	Jul. 31, 2014 USD ($)	Dec. 31, 2013
Line of Credit Facility [Line Items]
Outstanding Credit Facility balance			$ 573,000,000			$ 611,000,000
Credit Facility borrowings hedged						80,000,000
Consolidated Leverage Ratio Under Credit Facility & Note Payable, maximum		3.50%
Long-term Debt			597,100,000			593,664,000
Fair value of mortgage at inception			593,700,000			$ 609,500,000
Interest paid		$ 31,000,000	27,200,000	$ 12,300,000
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			150,000,000				$ 75,000,000	$ 125,000,000
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		75,000,000	$ 75,000,000				$ 50,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.25%			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%			3.72%				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
Outdated Unsecured Short Term Revolving Credit Facility [Member]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			$ 573,000,000			$ 611,000,000
Credit Facility weighted average interest rates on outstanding balance			1.90%		1.95%	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			$ 50,000,000
Agreement Term				3 years
Uncommitted Shelf Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Term	15 years