Real Estate Intangibles (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Intangible Assets:
Gross historical cost	$ 1,092,367,000	$ 1,070,754,000
Accumulated amortization	749,847,000	571,008,000
Net book value	342,520,000	499,746,000
Weighted-average amortization period in years	18 years 9 months	16 years 8 months
Intangible Liabilities:
Gross historical cost	86,264,000	85,871,000
Accumulated amortization	39,147,000	34,434,000
Net book value	47,117,000	51,437,000
Weighted-average amortization period in years	13 years 2 months	14 years 4 months
Intangible Liability [Member]
Estimated Aggregate Amortization Expense For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2014	3,328,000
2015	6,498,000
2016	7,264,000
2017	5,725,000
2018	5,380,000
Thereafter	18,922,000
Intangible Asset [Member]
Estimated Aggregate Amortization Expense For Acquired Lease Intangibles Expected To Be Recognized [Line Items]
2014	40,573,000
2015	66,906,000
2016	31,821,000
2017	23,183,000
2018	20,761,000
Thereafter	159,276,000
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	950,256,000	937,357,000
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	61,753,000	55,939,000
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	61,593,000	59,165,000
Lease commissions [Member]
Intangible Assets:
Gross historical cost	18,765,000	18,293,000
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	79,555,000	76,381,000
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 6,709,000	$ 9,490,000