Subsequent Events (Details) (USD $)	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014	Jan. 31, 2015	Nov. 30, 2012	Nov. 30, 2014
Subsequent Event [Line Items]
Credit facility, maximum borrowing capacity						$ 1,600,000,000.0
Net proceeds from sale of common stock	185,321,000	290,699,000	201,279,000
Term loan
Subsequent Event [Line Items]
Credit facility, maximum borrowing capacity	833,000,000			783,000,000			833,000,000
Decrease in basis points				0.25%
Term loan | Subsequent Event
Subsequent Event [Line Items]
Decrease in basis points					0.20%
Facility fee					0.05%
At-the-market equity offering programs
Subsequent Event [Line Items]
Net proceeds from sale of common stock	2,800,000
At-the-market equity offering programs | Subsequent Event
Subsequent Event [Line Items]
Aggregate gross proceeds from shares sold					895,956
Net proceeds from sale of common stock					113,400,000