Incentive Plan Incentive Plan (Long-Term Incentive Units) (Details) - Long-Term Incentive Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares:
Unvested units, ending (in shares)	280,891	314,415
Units granted (in shares)	127,252
Units vested (in shares)	(143,898)
Units cancelled or expired (in shares)	(16,878)
Unvested units, beginning (in shares)	314,415
Weighted-Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 59.34
Granted (in dollars per share)	66.99
Vested (in dollars per share)	59.65
Cancelled (in dollars per share)	58.13
Ending balance (in dollars per share)	$ 62.72	$ 59.34
Share/unit compensation expense	$ 5.9	$ 12.6	$ 8.9
Capitalized expense related to construction and leasing activities	1.2	1.7	$ 1.6
Unearned compensation	$ 9.9	$ 9.3
Unearned compensation, period of recognition	863 days