Acquisitions, Dispositions and Mortgage Repayments (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 sqft		Dec. 31, 2011 sqft
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price			$ 158.9
Mortgage Notes Payable Assumed			(52.4)
Cash Consideration			102.7	[1]
Real Estate			155.5
Note Receivable Repayment			(3.5)
Non-controlling interests			3.6
APIC			1.5
Other			(2.0)
Square Footage			548,209
Fair value adjustments in mortgage notes payable			2.0
Real estate acquisitions [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price	104.6		153.8
Construction Mortgage Note Receivable Repayments	(9.9)
Mortgage Notes Payable Assumed	(4.9)		(52.4)
Cash Consideration	89.8	[2]	101.4	[1]
Real Estate	104.8		155.5
Note Receivable Repayment
Non-controlling interests
APIC
Other	(0.2)		(1.7)
Square Footage	394,064		548,209
Real estate acquisitions [Member] | South Dakota [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Jan. 20, 2012
Purchase Price	15.0
Construction Mortgage Note Receivable Repayments
Cash Consideration	15.0	[2]
Real Estate	15.0
Other
Square Footage	58,285
Real estate acquisitions [Member] | North Carolina [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Feb. 10, 2012
Purchase Price	6.4
Construction Mortgage Note Receivable Repayments
Cash Consideration	6.4	[2]
Real Estate	6.4
Other
Square Footage	23,312
Real estate acquisitions [Member] | Pennsylvania [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Mar. 16, 2012		Apr. 28, 2011
Purchase Price	1.1		1.9
Construction Mortgage Note Receivable Repayments
Mortgage Notes Payable Assumed
Cash Consideration	1.1	[2]	1.9	[1]
Real Estate	1.1		1.9
Note Receivable Repayment
Non-controlling interests
APIC
Other
Square Footage
Real estate acquisitions [Member] | Pennsylvania One [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Nov. 15, 2011
Purchase Price			3.6
Mortgage Notes Payable Assumed
Cash Consideration			3.6	[1]
Real Estate			3.6
Note Receivable Repayment
Non-controlling interests
APIC
Other
Square Footage
Real estate acquisitions [Member] | Texas [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	May 23, 2012
Purchase Price	10.7
Construction Mortgage Note Receivable Repayments	(9.9)
Cash Consideration	0.8	[2]
Real Estate	10.7
Other
Square Footage	76,484
Real estate acquisitions [Member] | Texas One [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Dec. 20, 2012
Purchase Price	30.6
Construction Mortgage Note Receivable Repayments
Cash Consideration	30.6	[2]
Real Estate	30.6
Other
Square Footage	66,095
Real estate acquisitions [Member] | Tennessee [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Oct. 09, 2012
Purchase Price	11.0
Construction Mortgage Note Receivable Repayments
Cash Consideration	11.0	[2]
Real Estate	11.0
Other
Square Footage	39,345
Real estate acquisitions [Member] | Washington [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Oct. 12, 2012
Purchase Price	9.4
Construction Mortgage Note Receivable Repayments
Cash Consideration	9.4	[2]
Real Estate	9.4
Other
Square Footage	47,225
Real estate acquisitions [Member] | Virginia [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price			148.3
Cash Consideration			95.9
Square Footage			548,209
Real estate acquisitions [Member] | Virginia One [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Jun. 30, 2011
Purchase Price			32.0
Mortgage Notes Payable Assumed
Cash Consideration			32.0	[1]
Real Estate			31.9
Note Receivable Repayment
Non-controlling interests
APIC
Other			0.1
Square Footage			142,015
Real estate acquisitions [Member] | Virginia Two [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Aug. 04, 2011	[3]
Purchase Price			26.2	[3]
Mortgage Notes Payable Assumed			(12.2)	[3]
Cash Consideration			14.0	[1],[3]
Real Estate			26.4	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.2)	[3]
Square Footage			87,816	[3]
Real estate acquisitions [Member] | Virginia Three [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Aug. 04, 2011	[3]
Purchase Price			43.4	[3]
Mortgage Notes Payable Assumed			(18.6)	[3]
Cash Consideration			24.8	[1],[3]
Real Estate			43.8	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.4)	[3]
Square Footage			142,856	[3]
Real estate acquisitions [Member] | Virginia Four [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Aug. 30, 2011	[3]
Purchase Price			14.0	[3]
Mortgage Notes Payable Assumed			(7.0)	[3]
Cash Consideration			7.0	[1],[3]
Real Estate			14.6	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.6)	[3]
Square Footage			59,240	[3]
Real estate acquisitions [Member] | Virginia Five [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Sep. 30, 2011	[3]
Purchase Price			14.9	[3]
Mortgage Notes Payable Assumed			(7.5)	[3]
Cash Consideration			7.4	[1],[3]
Real Estate			15.1	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.2)	[3]
Square Footage			42,957	[3]
Real estate acquisitions [Member] | Virginia Six [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Oct. 26, 2011	[3]
Purchase Price			11.3	[3]
Mortgage Notes Payable Assumed			(4.5)	[3]
Cash Consideration			6.8	[1],[3]
Real Estate			11.6	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.3)	[3]
Square Footage			41,882	[3]
Real estate acquisitions [Member] | Virginia Seven [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired			Oct. 26, 2011	[3]
Purchase Price			6.5	[3]
Mortgage Notes Payable Assumed			(2.6)	[3]
Cash Consideration			3.9	[1],[3]
Real Estate			6.6	[3]
Note Receivable Repayment				[3]
Non-controlling interests				[3]
APIC				[3]
Other			(0.1)	[3]
Square Footage			31,443	[3]
Real estate acquisitions [Member] | Iowa [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Date Acquired	Dec. 21, 2012
Purchase Price	20.4
Construction Mortgage Note Receivable Repayments
Mortgage Notes Payable Assumed	(4.9)
Cash Consideration	15.5	[2]
Real Estate	20.6
Other	(0.2)
Square Footage	83,318
Fair value adjustments in mortgage notes payable	0.3
Purchase of noncontrolling interest [Member]
Real Estate Acquisitions and Mortgage Note Financing [Line Items]
Purchase Price	5.1		5.1
Mortgage Notes Payable Assumed
Cash Consideration			1.3	[1]
Real Estate
Note Receivable Repayment			(3.5)
Non-controlling interests			3.6
APIC			1.5
Other			$ (0.3)
Square Footage

[1] Cash Consideration excludes receivables acquired and liabilities assumed in the acquisition.

[2] Cash Consideration excludes receivables acquired and liabilities assumed in the acquisitions.

[3] The mortgage notes payable assumed in these acquisitions do not reflect fair value adjustments totaling $2.0 million recorded by the Company upon acquisition (included in Other).