Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

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Fair Value of Assets and Liabilities - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair value of assets and liabilities
Asset impairment charges		$ 18,400	$ 1,300		$ 19,714	$ 58,476	$ 17,162
Real estate properties, net					$ 1,296,893	2,101,635
Credit concentration \| Total rents
Fair value of assets and liabilities
Concentration risk					8.90%
111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Asset impairment charges				$ 43,700
Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Asset impairment charges						14,700
Real estate properties held for sale
Fair value of assets and liabilities
Asset impairment charges	$ 18,400
Level 2 \| Market Approach Valuation Technique \| Real estate properties held for sale
Fair value of assets and liabilities
Fair value of assets	80,600	80,600	23,300
Level 2 \| Market Approach Valuation Technique \| Real estate properties held for sale \| 111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Fair value of assets				264,900
Level 2 \| Market Approach Valuation Technique \| Real estate properties held for sale \| Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Fair value of assets						83,500
Disposition Plan \| Principal Balance \| Nonrecurring
Fair value of assets and liabilities
Real estate properties, net	$ 99,000	$ 99,000	$ 24,600
Disposition Plan \| Principal Balance \| Nonrecurring \| 111 Monument Circle, 101-115 W. Washington Street and 100 East Wisconsin Avenue
Fair value of assets and liabilities
Real estate properties, net				$ 308,600
Disposition Plan \| Principal Balance \| Nonrecurring \| Parkshore Plaza, Cabot Business Park Land, 625 Crane Street and 111 Market Place
Fair value of assets and liabilities
Real estate properties, net						$ 98,200