Real Estate Properties and Accumulated Depreciation - Historical Cost and Accumulated Depreciation and Amortization (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Property
Balance—beginning of period	$ 883.6	$ 660.2	$ 498.4
Disposals	(8.5)	(1.2)	(1.2)
Impairments	(4.0)	(17.1)	0
Additions (acquisitions and improvements)	249.4	241.7	163.0
Balance, end of period	1,120.5	883.6	660.2
Accumulated Depreciation
Balance—beginning of period	176.7	131.2	94.7
Disposals	(9.3)	(1.2)	(1.2)
Impairments	(0.9)	(5.3)	0
Additions (depreciation and amortization expense)	70.2	52.0	37.7
Balance, end of period	$ 236.7	$ 176.7	$ 131.2