Incentive Plan (Summary of Restricted Stock Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Units
Granted (shares)			70,082
Weighted-Average Grant Date Fair Value
Award vesting period			4 years
Restricted Stock
Units
Unvested beginning of period (shares)			274,642
Granted (shares)			117,671
Vested (shares)			(96,123)
Cancelled or expired (shares)			(26,955)
Unvested end of period (shares)	269,235		269,235
Weighted-Average Grant Date Fair Value
Unvested, beginning of period (in dollars per share)			$ 73.81
Granted (in dollars per share)			108.39
Vested (in dollars per share)			68.80
Cancelled or expired (in dollars per share)			84.22
Unvested, end of period (in dollars per share)	$ 89.67		$ 89.67
Restricted stock expense	$ 1.1	$ 1.1	$ 3.3	$ 3.1
Capitalized expense related to construction and leasing activities	0.8	$ 0.6	2.5	$ 2.2
Unearned compensation	$ 19.5		$ 19.5		$ 14.7
Unearned compensation, period of recognition (in years)			2 years 9 months 5 days
Restricted Stock | Maximum
Weighted-Average Grant Date Fair Value
Award vesting period			4 years