Subsequent Event (Details) (Subsequent events, Unsecured Euro-denominated term loan facility)	1 Months Ended
	Jul. 31, 2012 USD ($) item	Jul. 31, 2012 EUR (€)
Subsequent events
Maximum borrowing capacity under credit agreement	$ 463,200,000	€ 365,000,000
Maximum number of tranches in which facility can be drawn	5
Variable rate basis	EURIBOR
Applicable margin (as a percent)	3.00%
Commitment fee on undrawn amount (as a percent)	1.05%
Prepayment of unsecured floating rate term loan	$ 100,000,000