Real Estate Properties and Accumulated Depreciation - Historical Cost and Accumulated Depreciation and Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property
Balance—beginning of period	$ 1,378.4	$ 1,120.5	$ 883.6
Disposals	(7.0)	(0.1)	(8.5)
Impairments	(9.3)	0.0	(4.0)
Additions (acquisitions and improvements)	465.5	258.0	249.4
Balance, end of period	1,827.6	1,378.4	1,120.5
Accumulated Depreciation
Balance—beginning of period	327.0	236.7	176.7
Disposals	(2.7)	0.0	(9.3)
Impairments	0.0	0.0	(0.9)
Additions (depreciation and amortization expense)	111.3	90.3	70.2
Balance, end of period	$ 435.6	$ 327.0	$ 236.7