Incentive Plan (Restricted Stock Activity) (Details) - Restricted Stock - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Shares:
Unvested units, beginning (in shares)	302,298
Units granted (in shares)	109,780
Units vested (in shares)	(99,988)
Units cancelled or expired (in shares)	(41,376)
Unvested units, ending (in shares)	270,714	302,298
Weighted-Average Grant Date Fair Value
Beginning balance (in dollars per share)	$ 57.10
Granted (in dollars per share)	66.90
Vested (in dollars per share)	59.57
Cancelled (in dollars per share)	56.85
Ending balance (in dollars per share)	$ 60.20	$ 57.10
Additional Information:
Restricted stock expense	$ 2.5	$ 2.5	$ 2.7
Capitalized expense related to construction and leasing activities	2.7	2.7	$ 2.5
Compensation not yet recognized	$ 10.4	$ 10.4
Unearned compensation, period of recognition	883 days
Minimum
Additional Information:
Award vesting period	3 years
Maximum
Additional Information:
Award vesting period	4 years