Assets Held for Sale (Details) $ in Thousands, ft² in Millions		3 Months Ended		9 Months Ended
	Oct. 03, 2016 USD ($) ft² a bldg	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Buildings and Land				$ 142,808	$ 243,943
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Total assets held for sale		$ 786,272		786,272		$ 4,954
Total liabilities held for sale		25,416		25,416		$ 0
October 2016 Sale [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Net income		20,524	$ 14,595	48,182	45,126
Net income available to common shareholders		20,041	$ 14,252	47,050	$ 44,079
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land and Land Improvements		202,641		202,641
Building and Building Improvements		834,286		834,286
Less accumulated depreciation		(319,495)		(319,495)
Operating real estate		717,432		717,432
Land held for development		15,524		15,524
Accounts receivable, net		1,358		1,358
Deferred rent receivable, net		22,140		22,140
Deferred financing and leasing costs, net		23,493		23,493
Prepaid expenses and other assets		3,595		3,595
Total assets held for sale		783,542		783,542
Accounts payable		8,650		8,650
Other liabilities		16,766		16,766
Total liabilities held for sale		25,416		25,416
2016 held for sale [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract]
Land held for development		$ 2,700		$ 2,700
Subsequent Event [Member] | October 2016 Sale [Member] | Disposal Group, Held-for-sale, Not Discontinued Operations [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties | bldg	108
Area of Real Estate Property | ft²	7.6
Area of Land | a	26.7
Proceeds from Sale of Buildings and Land	$ 969,000