Incentive Plan (Summary of Long-Term Incentive Units) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Weighted-Average Grant Date Fair Value
Award vesting period			4 years
Long-Term Incentive Units
Units
Unvested beginning of period (shares)			276,669
Granted (shares)			24,823
Vested (shares)			(134,318)
Cancelled or expired (shares)			0
Unvested end of period (shares)	167,174		167,174
Weighted-Average Grant Date Fair Value
Unvested beginning of period ($ per share)			$ 62.92
Granted ($ per share)			84.65
Vested ($ per share)			62.74
Cancelled or expired ($ per share)			0.00
Unvested end of period ($ per share)	$ 66.28		$ 66.28
Award vesting period			4 years
Share/unit compensation expense	$ 2.5	$ 1.8	$ 3.5	$ 3.1
Capitalized expense related to construction and leasing activities	0.7	$ 0.1	1.2	$ 1.0
Unearned compensation	$ 8.3		$ 8.3		$ 9.9
Unearned compensation, period of recognition			2 years 3 months 7 days