Summary of Significant Accounting Policies - Real Estate and Depreciation, In-place Leases (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 32,093	$ 31,991
Accumulated Amortization	22,336	19,293
Net	9,757	12,698
Amortization	20,636	15,360	7,643
Leases, Acquired-in-Place [Member]
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2013	17,382
2014	14,837
2015	12,228
2016	8,984
2017	5,548
Tenant origination costs [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	58,444	55,640
Accumulated Amortization	32,839	25,479
Net	25,605	30,161
Leasing commissions and absorbtion costs [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	90,327	86,705
Accumulated Amortization	48,163	34,738
Net	42,164	51,967
Net lease intangible assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	14,794	14,422
Accumulated Amortization	7,665	5,679
Net	7,129	8,743
Below-market ground lease [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	12,080	12,080
Accumulated Amortization	956	766
Net	$ 11,124	$ 11,314