Incentive Plan (Restricted Stock Activity) (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
Restricted Stock
Shares:
Unvested, beginning of period (in shares)	274,642
Granted (in shares)	120,853
Vested (in shares)	(102,899)
Cancelled or expired (in shares)	(33,174)
Unvested, end of period (in shares)	259,422	274,642
Weighted-Average Grant Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 73.81
Granted (in dollars per share)	108.65
Vested (in dollars per share)	69.04
Cancelled or expired (in dollars per share)	84.68
Unvested, end of period (in dollars per share)	$ 90.54	$ 73.81
Additional Information:
Restricted stock expense	$ 4.5	$ 4.1	$ 2.5
Capitalized expense related to construction and leasing activities	3.3	2.8	$ 2.7
Compensation not yet recognized	$ 17.5	$ 14.7
Unearned compensation, period of recognition	2 years 8 months 12 days
Restricted Stock | Minimum
Additional Information:
Award vesting period	3 years
Restricted Stock | Maximum
Additional Information:
Award vesting period	4 years