Related Party Transactions (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013 sqft	Sep. 30, 2012	Sep. 30, 2013 contract sqft	Sep. 30, 2012	Dec. 31, 2012 contract sqft	Dec. 31, 2006 contract
Related Party Transaction [Line Items]
Rental revenues	$ 290,712,000	$ 260,052,000	$ 858,064,000	$ 717,809,000
Tel(x) [Member]
Related Party Transaction [Line Items]
Number of leases agreement			51		44	10
Lease expiration, date			2026
Terms of lease extended through			2046
Rental revenues	15,600,000	12,000,000	44,000,000	33,700,000
Square footage of net rentable space	331,002		331,002		288,940
Percentage rent lease revenue earned	2,000,000	1,700,000	3,900,000	3,100,000
Percentage of rent for management fee			1.00%
Unrelated Third Party [Member]
Related Party Transaction [Line Items]
Number of leases agreement			10
Square footage of net rentable space	76,684		76,684
SoftLayer [Member]
Related Party Transaction [Line Items]
Number of leases agreement			9
Rental revenues	$ 14,900,000	$ 19,400,000	$ 40,600,000	$ 41,200,000
Minimum [Member] | SoftLayer [Member]
Related Party Transaction [Line Items]
Lease expiration, date			2013
Lease option extension date			2018
Maximum [Member] | SoftLayer [Member]
Related Party Transaction [Line Items]
Lease expiration, date			2025
Lease option extension date			2035