Related Person Transactions - Narrative (Details) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended		12 Months Ended
	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Mar. 31, 2017 USD ($) option	Mar. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)	Jul. 31, 2014 USD ($) $ / shares
Minimum
Related Party Transaction [Line Items]
Average closing price per common share during one year period after date (at least) (in dollars per share) | $ / shares						$ 26.00
Two North Riverside Plaza Joint Venture Limited Partnership
Related Party Transaction [Line Items]
Lease term			5 years
Number of renewal options of lease arrangement | option			1
Renewal term of lease arrangement			5 years
Tenant Improvements			$ 0.7
Lease termination period duration			30 days
Expenses from transactions with related party			$ 0.2	$ 0.2
Related/Corvex
Related Party Transaction [Line Items]
Due to related persons						$ 33.5
Amount of reimbursements payable to related party upon the approval of shareholders	$ 8.2	$ 8.4			$ 16.7
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)						8.4
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)						$ 8.4
Related/Corvex | Minimum
Related Party Transaction [Line Items]
Average closing price per common share during one year period after date (at least) (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