Incentive Plan (Restricted Stock Activity) (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shares:
Unvested, beginning of period (in shares)	271,901
Granted (in shares)	162,685
Vested (in shares)	(120,169)
Cancelled or expired (in shares)	(39,775)
Unvested, end of period (in shares)	274,642	271,901
Weighted-Average Grant Date Fair Value
Unvested, beginning of period (in dollars per share)	$ 61.47
Granted (in dollars per share)	83.58
Vested (in dollars per share)	61.30
Cancelled or expired (in dollars per share)	67.22
Unvested, end of period (in dollars per share)	$ 73.81	$ 61.47
Additional Information:
Restricted stock expense	$ 4.1	$ 2.5	$ 2.5
Capitalized expense related to construction and leasing activities	2.8	2.7	$ 2.7
Compensation not yet recognized	$ 14.7	$ 10.4
Unearned compensation, period of recognition	2 years 10 months 12 days
Minimum
Additional Information:
Award vesting period	3 years
Maximum
Additional Information:
Award vesting period	4 years