Summary of Significant Accounting Policies (Impairment or Disposal of Long-Lived Assets/ Revenue Recognition/ Stock-Based Compensation Plans) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Jun. 02, 2010
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Development expense for land	$ 7.7
Revenue increased by straight-line rent adjustment	17.9	10.9	6.4
Increased revenue due to amortization of deferred rent	2.4	2.8	2.7
Decrease in revenue due to amortization of lease incentives	1.6	1.5	1.8
Additional income related to electricity charges	0.5
Overstatement of revenue due to under billing	(0.5)
Understatement of Revenue			0.3	0.2
Increase in Common shares available for future awards					6,000,000
Increase in shares due to amendments in plan, shares solely available for options and share appreciation rights	3,971,774				3,600,000
Common shares available for future awards	5,789,202
Options awarded, term	one to ten-year
Award vesting term, minimum	1 year
Award vesting term, maximum	10 years
Stock-based compensation expense	6.4	6.1	5.3
Stock-based compensation expense, capitalized	$ 1.5	$ 1.3	$ 0.8
Requirement to commence development by 2012 [Member]
Summary Of Significant Accounting Policies (Textuals) [Abstract]
Number of parcels of land under option for ground lease	2