Summary of Significant Accounting Policies Deferred Leasing and Other Costs (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Deferred Leasing and Other [Abstract]
Number of Days Past Due	90
Amortization of Intangible Assets	$ 27,200,000	$ 33,700,000	$ 38,700,000
Deferred Costs, Leasing, Gross	312,206,000	275,491,000
Finite-Lived Intangible Asset, Acquired-in-Place Leases	189,927,000	170,048,000
Deferred Leasing Costs and Finite Lived Intangible Assets Acquired, gross	502,133,000	445,539,000
Deferred Costs, Leasing, Accumulated Amortization	(108,177,000)	(95,343,000)
Finite-Lived Intangible Assets, Accumulated Amortization	(101,274,000)	(91,455,000)
Deferred Costs, Leasing, Net	292,682,000	258,741,000
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	21,904,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two	16,455,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three	12,700,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four	9,563,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five	7,080,000
Finite-Lived Intangible Assets, Amortization Expense, after Year Five	17,701,000
Finite-Lived Intangible Assets, Net	85,403,000
Net Charge to Rental Income Year One	777,000
Net Charge to Rental Income Year Two	703,000
Net Charge to Rental Income Third Year	639,000
Net Charge to Rental Income Year Four	367,000
Net Charge to Rental Income Year Five	353,000
Net Charge to Rental Income After Year Five	411,000
Net Charge to Rental Income	3,250,000
Amortization of above market lease, charge to rental income	$ 913,000	$ 1,000,000	$ 1,600,000