Incentive Plan (Summary of Restricted Stock Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Units
Granted (shares)	55,324
Weighted-Average Grant Date Fair Value
Award vesting period	4 years
Restricted Stock
Units
Unvested beginning of period (shares)	274,642
Granted (shares)	107,411
Vested (shares)	(86,209)
Cancelled or expired (shares)	(4,366)
Unvested end of period (shares)	291,478
Weighted-Average Grant Date Fair Value
Unvested, beginning of period ($ per share)	$ 73.81
Granted ($ per share)	108.00
Vested ($ per share)	67.27
Cancelled or expired ($ per share)	79.75
Unvested, end of period ($ per share)	$ 88.25
Restricted stock expense	$ 1.0	$ 0.9
Capitalized expense related to construction and leasing activities	0.8	$ 0.8
Unearned compensation	$ 24.3		$ 14.7
Unearned compensation, period of recognition (in years)	3 years 3 months 2 days
Restricted Stock | Maximum
Weighted-Average Grant Date Fair Value
Award vesting period	4 years