Represents the compensation cost capitalized during the period arising from equity-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Incentive Plan (Summary of Restricted Stock Activity) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Weighted-Average Grant Date Fair Value
Award vesting period			4 years
Restricted Stock
Units
Unvested beginning of period (shares)			271,339
Granted (shares)			150,480
Vested (shares)			(114,834)
Cancelled or expired (shares)			(31,997)
Unvested end of period (shares)	274,988		274,988
Weighted-Average Grant Date Fair Value
Unvested, beginning of period ($ per share)			$ 61.37
Granted ($ per share)			82.88
Vested ($ per share)			61.46
Cancelled or expired ($ per share)			66.65
Unvested, end of period ($ per share)	$ 72.61		$ 72.61
Restricted stock expense	$ 1.1	$ 0.7	$ 3.1	$ 1.8
Capitalized expense related to construction and leasing activities	0.6	$ 0.6	2.2	$ 2.1
Unearned compensation	$ 15.6		$ 15.6		$ 10.4
Unearned compensation, period of recognition			2 years 11 months 20 days
Restricted Stock \| Maximum
Weighted-Average Grant Date Fair Value
Award vesting period			4 years