Real Estate Intangibles (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Assets:
Gross historical cost	$ 1,050,542,000	$ 673,684,000
Accumulated amortization	(472,389,000)	(257,242,000)
Net book value	578,153,000	416,442,000
Weighted-average amortization period in years	17 years 1 month	16 years 4 months
Liabilities:
Gross historical cost	85,729,000	86,526,000
Accumulated amortization	(32,894,000)	(27,753,000)
Net book value	52,835,000	58,773,000
Weighted-average amortization period in years	14 years 3 months	14 years 3 months
Intangible Liability [Member]
Estimated Aggregate Amortization Expense For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2013	1,730,000
2014	6,583,000
2015	5,623,000
2016	5,185,000
2017	4,894,000
Thereafter	28,820,000
Totals	52,835,000
Intangible Asset [Member]
Estimated Aggregate Amortization Expense For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2013	93,415,000
2014	207,606,000
2015	58,816,000
2016	25,985,000
2017	26,825,000
Thereafter	165,506,000
Totals	578,153,000
In place lease intangibles [Member]
Assets:
Gross historical cost	918,507,000	541,729,000
Above market tenant leases [Member]
Assets:
Gross historical cost	54,091,000	56,086,000
Below market ground leases [Member]
Assets:
Gross historical cost	61,461,000	61,450,000
Lease commissions [Member]
Assets:
Gross historical cost	16,483,000	14,419,000
Below market tenant leases [Member]
Liabilities:
Gross historical cost	76,239,000	77,036,000
Above market ground leases [Member]
Liabilities:
Gross historical cost	$ 9,490,000	$ 9,490,000