Debt (Details) (USD $)			9 Months Ended		9 Months Ended						9 Months Ended			9 Months Ended			0 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 SV1 - Mortgage loan	Dec. 31, 2012 SV1 - Mortgage loan	Sep. 30, 2013 Revolving credit facility item	Jun. 28, 2013 Revolving credit facility	Dec. 31, 2012 Revolving credit facility	Sep. 30, 2013 Revolving credit facility Operating partnership	Sep. 30, 2013 Revolving credit facility Minimum	Sep. 30, 2013 Revolving credit facility Maximum	Sep. 30, 2013 Revolving credit facility LIBOR Operating partnership	Sep. 30, 2013 Revolving credit facility LIBOR Minimum Operating partnership	Sep. 30, 2013 Revolving credit facility LIBOR Maximum Operating partnership	Sep. 30, 2013 Revolving credit facility Base rate Operating partnership	Sep. 30, 2013 Revolving credit facility Base rate Minimum Operating partnership	Sep. 30, 2013 Revolving credit facility Base rate Maximum Operating partnership	Jun. 28, 2013 Prior facility	Jan. 02, 2013 Prior facility	Sep. 30, 2013 Prior facility Operating partnership
Debt
Interest rate as of reporting date (as a percent)			3.69%	3.71%	2.18%		2.46%
Total principal outstanding	$ 166,625,000	$ 59,750,000	$ 58,625,000	$ 59,750,000	$ 108,000,000
Commitment under the credit agreement					405,000,000	355,000,000		500,000,000									355,000,000	225,000,000
Number of options available for extensions					1
Extension fee (as a percent)					0.25%
Total increase in commitment under accordion feature																	50,000,000		145,000,000
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					396,600,000
Outstanding borrowings					108,000,000
Leverage ratio, Covenant (as a percent)										60.00%
Leverage ratio (as a percent)					11.50%
Secured debt ratio, Covenant (as a percent)										3.80%
Secured debt ratio (as a percent)					4.00%
Fixed charge coverage ratio, Covenant									1.75
Fixed charge coverage ratio									750.00%	100.00%
Unhedged variable rate debt ratio, Covenant (as a percent)										30.00%
Unhedged variable rate debt ratio (as a percent)					10.80%
Repayment of senior mortgage loan			$ 58,600,000