Incentive Plan Incentive Plan (Long-Term Incentive Units) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares:
Granted (in shares)	73,449	1,416
Long-Term Incentive Units
Shares:
Unvested, beginning of period (in shares)	128,822
Granted (in shares)	72,664
Vested (in shares)	(94,801)
Cancelled or expired (in shares)	(7,390)
Unvested, end of period (in shares)	99,295	128,822
Weighted-Average Grant Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 66.58
Granted (in dollars per share)	109.71
Vested (in dollars per share)	71.97
Cancelled (in dollars per share)	98.94
Unvested, end of period (in dollars per share)	$ 90.59	$ 66.58
Share/unit compensation expense	$ 3.9	$ 4.5	$ 5.9
Capitalized expense related to construction and leasing activities	1.7	1.8	$ 1.2
Unearned compensation	$ 6.9	$ 5.6
Unearned compensation, period of recognition	2 years 6 months 20 days