Real Estate Intangibles (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended	3 Months Ended
	Sep. 30, 2012 year	Dec. 31, 2011 year	Sep. 30, 2012 Intangible Liability [Member]	Sep. 30, 2012 Intangible Asset [Member]	Sep. 30, 2012 In place lease intangibles [Member]	Dec. 31, 2011 In place lease intangibles [Member]	Sep. 30, 2012 Above market tenant leases [Member]	Dec. 31, 2011 Above market tenant leases [Member]	Sep. 30, 2012 Below market ground leases [Member]	Dec. 31, 2011 Below market ground leases [Member]	Sep. 30, 2012 Lease commissions [Member]	Dec. 31, 2011 Lease commissions [Member]	Sep. 30, 2012 Below market tenant leases [Member]	Dec. 31, 2011 Below market tenant leases [Member]	Sep. 30, 2012 Above market ground leases [Member]	Dec. 31, 2011 Above market ground leases [Member]
Assets:
Gross historical cost	$ 572,765	$ 428,199			$ 447,833	$ 332,645	$ 52,210	$ 35,973	$ 60,771	$ 51,316	$ 11,951	$ 8,265
Accumulated amortization	230,217	148,380
Net book value	342,548	279,819
Weighted-average amortization period in years	19 years 6 months	17 years 0 months
Liabilities:
Gross historical cost	81,397	72,304											72,442	67,284	8,955	5,020
Accumulated amortization	26,010	21,387
Net book value	55,387	50,917
Weighted-average amortization period in years	14.5	12.3
Estimated Aggregate Amortization Expense For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2012			1,742	51,651
2013			6,534	38,021
2014			6,010	32,294
2015			5,014	22,877
2016			4,668	24,184
Thereafter			31,419	173,521
Totals			$ 55,387	$ 342,548