Related Party Transactions (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013 contract sqft	Mar. 31, 2012	Dec. 31, 2012 contract sqft	Dec. 31, 2006 contract
Related Party Transaction [Line Items]
Rental revenues	$ 281,399,000	$ 222,834,000
Tel(x) [Member]
Related Party Transaction [Line Items]
Number of leases agreement	50		44	10
Lease expiration, date	2026
Terms of lease extended through	2046
Rental revenues	16,000,000	10,400,000
Square footage of net rentable space	323,626		288,940
Percentage rent lease revenue earned	500,000	300,000
Percentage of rent for management fee	1.00%
Unrelated Third Party [Member]
Related Party Transaction [Line Items]
Number of leases agreement	9
Square footage of net rentable space	69,312
SoftLayer [Member]
Related Party Transaction [Line Items]
Number of leases agreement	9
Rental revenues	$ 10,900,000	$ 7,300,000
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