Debt (Details) (USD $)			12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended			12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 SV1 - Mortgage loan	Dec. 31, 2011 SV1 - Mortgage loan	Oct. 07, 2010 SV1 - Mortgage loan Interest rate swap	Dec. 31, 2012 Revolving credit facility item	Jan. 03, 2013 Revolving credit facility	Dec. 31, 2011 Revolving credit facility	Dec. 31, 2012 Revolving credit facility Operating partnership	Dec. 31, 2012 Revolving credit facility Minimum	Dec. 31, 2012 Revolving credit facility Maximum	Dec. 31, 2012 Revolving credit facility LIBOR Operating partnership	Dec. 31, 2012 Revolving credit facility LIBOR Minimum Operating partnership	Dec. 31, 2012 Revolving credit facility LIBOR Maximum Operating partnership	Dec. 31, 2012 Revolving credit facility Base rate Operating partnership	Dec. 31, 2012 Revolving credit facility Base rate Minimum Operating partnership	Dec. 31, 2012 Revolving credit facility Base rate Maximum Operating partnership	Dec. 31, 2011 CH1 - Senior mortgage loan	Dec. 31, 2011 VA1- Mortgage loan	Dec. 31, 2012 Prior facility	Dec. 31, 2012 Prior facility Operating partnership
Debt
Interest rate as of reporting date (as a percent)			3.71%	3.75%				2.54%										0.88%
Total principal outstanding	$ 59,750,000	$ 121,864,000	$ 59,750,000	$ 60,000,000				$ 5,000,000										$ 25,000,000	$ 31,864,000
Commitment under the credit agreement							355,000,000													225,000,000
Number of options available for extensions						1
Extension fee (as a percent)						0.25%
Total commitment under accordion feature									500,000,000												145,000,000
Outstanding borrowings						0		5,000,000
Reference rate												LIBOR			base rate
Interest rate margin (as a percent)													2.00%	2.75%		1.00%	1.75%
Amount available for borrowing under the facility							346,500,000
Leverage ratio (as a percent)											60.00%
Secured debt ratio (as a percent)											40.00%
Fixed charge coverage ratio										1.75
Unhedged variable rate debt ratio (as a percent)											30.00%
Period of extension of maturity			2 years
Notional amount of interest rate derivatives					$ 60,000,000