Fair Value of Assets and Liabilities - Narrative (Details)	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) Tenant	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Jul. 31, 2014 USD ($) $ / shares
Fair value of assets and liabilities
Loss on asset impairment			$ 0	$ 0	$ 43,736,000	$ 17,162,000
Number of tenants responsible for more than 4% of total annualized rents | Tenant					0
Real estate properties, net			2,183,931,000		$ 2,183,931,000			$ 2,988,413,000
Total rents | Credit concentration
Fair value of assets and liabilities
Concentration risk, percent					5.50%
General and Administrative Expense
Fair value of assets and liabilities
Change in fair value of derivatives			(100,000)		$ 1,000,000
Market Approach Valuation Technique | Real Estate Held-for-sale | Level 2
Fair value of assets and liabilities
Assets, fair value disclosure			264,900,000		264,900,000
Disposition Plan | Carrying Amount | Fair Value, Measurements, Nonrecurring
Fair value of assets and liabilities
Real estate properties, net			308,600,000		308,600,000
Derivative liability | Level 3
Fair value of assets and liabilities
Derivative liability			0		0			$ 7,200,000
Minimum
Fair value of assets and liabilities
Average closing price per common share during one year period after date (in dollars per share) | $ / shares									$ 26.00
Related/Corvex
Fair value of assets and liabilities
Amount of reimbursements payable to related party upon the approval of shareholders	$ 8,200,000	$ 8,400,000					$ 16,700,000
Amount of reimbursements payable to related party if specified minimum common share price maintain during the one year period after the date on which the reimbursement is approved by shareholders (up to)									$ 8,400,000
Amount of reimbursements payable to related party if specified minimum common share price maintain during the one year period between the first and second anniversaries of the date on which the reimbursement is approved by shareholders (up to)									$ 8,400,000
Derivative liability, potential reimbursement to related party contingent on stock price			$ 8,400,000		$ 8,400,000
Related/Corvex | Minimum
Fair value of assets and liabilities
Average closing price per common share during one year period after date (in dollars per share) | $ / shares									$ 26.00
Average closing price per common share during first and second year after date (in dollars per share) | $ / shares									$ 26.00