Real Estate Intangibles (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Intangible Assets:
Gross historical cost	$ 1,208,510,000	$ 1,135,936,000
Accumulated amortization	836,684,000	776,501,000
Net book value	$ 371,826,000	$ 359,435,000
Weighted-average amortization period in years	17 years 2 months	17 years 8 months
Intangible Liabilities:
Gross historical cost	$ 97,751,000	$ 99,027,000
Accumulated amortization	44,609,000	40,891,000
Net book value	$ 53,142,000	$ 58,136,000
Weighted-average amortization period in years	15 years 7 months	14 years 5 months
In place lease intangibles [Member]
Intangible Assets:
Gross historical cost	$ 1,042,770,000	$ 988,290,000
Above market tenant leases [Member]
Intangible Assets:
Gross historical cost	82,083,000	65,684,000
Below market ground leases [Member]
Intangible Assets:
Gross historical cost	61,159,000	62,426,000
Lease commissions [Member]
Intangible Assets:
Gross historical cost	22,498,000	19,536,000
Below market tenant leases [Member]
Intangible Liabilities:
Gross historical cost	89,891,000	91,168,000
Above market ground leases [Member]
Intangible Liabilities:
Gross historical cost	$ 7,860,000	$ 7,859,000