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		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 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)			136,566
Vested (shares)			(101,181)
Cancelled or expired (shares)			(22,325)
Unvested end of period (shares)	284,399		284,399
Weighted-Average Grant Date Fair Value
Unvested beginning of period ($ per share)			$ 61.37
Granted ($ per share)			80.96
Vested ($ per share)			61.07
Cancelled or expired ($ per share)			65.93
Unvested end of period ($ per share)	$ 70.54		$ 70.54
Restricted stock expense	$ 1.2	$ 0.7	$ 2.1	$ 1.2
Capitalized expense related to construction and leasing activities	0.8	$ 0.8	1.5	$ 1.5
Unearned compensation	$ 16.7		$ 16.7		$ 10.4
Unearned compensation, period of recognition			3 years 9 days
Restricted Stock \| Maximum
Weighted-Average Grant Date Fair Value
Award vesting period			4 years