Related Person Transactions - Narrative (Details)														1 Months Ended			3 Months Ended	5 Months Ended	6 Months Ended	9 Months Ended	12 Months Ended
	Jul. 20, 2015 option	Mar. 01, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($) trustee	Sep. 29, 2014 trustee agreement	Jul. 09, 2014 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) ft² shares	Jun. 01, 2014 option parking_space	May 09, 2014 USD ($) company shares	Mar. 01, 2014 USD ($) property	Dec. 19, 2013 USD ($)	Mar. 15, 2013 shares	Feb. 29, 2012	Aug. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Mar. 31, 2012 USD ($) shares	Jun. 30, 2014 ft² office shares	Feb. 28, 2015 USD ($)	Jun. 30, 2014 USD ($) ft² shares	Sep. 30, 2014 USD ($)	Dec. 31, 2016 USD ($) trustee	Dec. 31, 2015 USD ($) installment	Dec. 31, 2014 USD ($)	Dec. 31, 2009 trustee	Jul. 31, 2014 USD ($) $ / shares	May 31, 2014 USD ($)	May 08, 2014	Mar. 31, 2014 ft²	Jan. 31, 2013 trustee
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Property square feet | ft²																												5,641,450
SIR
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Number of former managing trustees | trustee																					2
Number of former independent trustees | trustee																					1
Prior ownership percentage													100.00%
Shares sold (in shares) | shares																9,200,000
Less than wholly owned subsidiary, common shares owned (in shares) | shares							22,000,000										22,000,000		22,000,000
Proceeds from IPO																$ 180,800,000
Equity investments, number of shares sold (in shares) | shares						22,000,000
Price per common share (in dollars per share) | $ / shares						$ 32.04
Equity investments, proceeds from sale						$ 704,800,000
Ownership percentage						0.00%	36.70%										36.70%		36.70%
Parent Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Number of properties | property										1
Maximum
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Ownership percentage																					50.00%
Two North Riverside Plaza Joint Venture Limited Partnership
Related Party Transaction [Line Items]
Lease term	5 years
Number of renewal options | option	1
Renewal option term	5 years
Tenant Improvements																					$ 700,000
Expenses from transactions with related party																					800,000	$ 13,000	$ 100,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2017																					800,000
2018																					900,000
2019																					900,000
2020																					900,000
Due to related party																					$ 100,000
Equity Group
Related Party Transaction [Line Items]
Lease term								1 year
Number of renewal options | option								1
Renewal option term								3 months
Equity Group | Terminated Agreement [Member]
Related Party Transaction [Line Items]
Expenses from transactions with related party																						200,000	100,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Number of parking spaces | parking_space								1
Equity Residential Management, L.L.C.
Related Party Transaction [Line Items]
Expenses from transactions with related party																						$ 400,000	38,000
Related/Corvex
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Amount of reimbursements payable to related party																									$ 33,500,000
Reimbursement amount paid														$ 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																									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																									$ 8,400,000
Related/Corvex | Minimum
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Price per common share during one year period after date (in dollars per share) | $ / shares																									$ 26.00
Price per common share during first and second year after date (in dollars per share) | $ / shares																									$ 26.00
RMR
Related Party Transaction [Line Items]
Lease term																	3 years
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Number of agreements | agreement					3
Number of former managing trustees | trustee					1
Number of former independent trustees | trustee				2
Business management fees																			$ 31,700,000
Shares issued (in shares) | shares																			68,206
Property management fees as a percentage of gross collected rents							3.00%										3.00%		3.00%
Construction supervision fees as a percentage of construction costs							5.00%										5.00%		5.00%
Property management and construction supervision fees																			$ 13,800,000
Number of regional offices leased | office																	11
Rental income earned																			$ 400,000
Property square feet | ft²							36,500										36,500		36,500
Legal fees and costs																			$ 5,400,000
Number of properties | property										2
Total rent due										$ 1,100,000
RMR | Parent Company
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Number of properties acquired or agreed to be acquired | property										1
RMR | Amended Agreement
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Base management fee payable as a percentage of average historical cost of real estate investments, up to specified amount											0.70%
Base management fee payable as a percentage of average historical cost of real estate investments, located outside the United States, Puerto Rico and Canada											1.00%
Base management fee payable as a percentage of average historical cost of real estate investments, exceeding specified amount											0.50%
Base management fee payable as a percentage of average closing price per share of common shares on NYSE											0.70%
Base management fee payable as a percentage of average market capitalization exceeding specified amount											0.50%
Base business management fee payable in cash											90.00%
Base business management fee payable in common shares							10.00%				10.00%						10.00%		10.00%
Minimum ownership of outstanding common shares of related party required to engage the same manager or common managing trustees											10.00%
Percentage for limitation and adjustments of incentive management fee payable											12.00%
Measurement period of incentive management fee											3 years
Incentive management fee vesting on the date of issuance, percent											33.33%
Incentive management fee vesting one year from the date of issuance, percent											33.34%
Incentive management fee vesting in two years from the date of issuance, percent											33.33%
RMR | Amended Agreement | 2014
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Measurement period of incentive management fee																							1 year
RMR | Amended Agreement | 2015
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Measurement period of incentive management fee																						2 years
RMR | Maximum | Amended Agreement
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Threshold amount of real estate investments for payment of base management fee											$ 250,000,000
Base management fee payable, average market capitalization											$ 250,000,000
RMR Australia
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Property management fees as a percentage of gross collected rents																													3.00%
Construction supervision fees as a percentage of construction costs																													5.00%
Number of former trustees owning subsidiary | trustee																													2
Compensation for business management and real estate investment services as a percentage of average historical cost of CWHAT's real estate investments																													0.50%
Compensation for property management services as a percentage of difference between 3.5% of collected gross rents and the aggregate of all amounts paid or payable by or on behalf of CWHAT to third party property managers																													50.00%
Compensation for construction supervision services as a percentage of difference between 5.0% of construction costs and any amounts paid to third parties for construction management and/or supervision																													50.00%
Business and property management fees																			$ 900,000
RMR and RMR Australia
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Monthly management fees		$ 100,000																$ 1,200,000
Pro rata portion of the current year's incentive fee under the existing business management agreement																				$ 15,300,000
GOV
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Prior ownership percentage																								100.00%
Number of former trustees currently managing trustees | trustee																								2
Shares sold (in shares) | shares												9,950,000
AIC
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Equity investments, number of shares sold (in shares) | shares									20,000
Equity investments, proceeds from sale									$ 5,800,000
Ownership percentage									0.00%																		12.50%
Amount invested in equity investee									$ 5,200,000
Number of other companies which are shareholders of related party | company									4
Coverage amount for property insurance																										$ 500,000,000.0
Premium for property insurance																			$ 6,000,000
Equity Commonwealth and REIT Management and Research LLC
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Settlement receivable										$ 12,000,000
Number of settlement receivable installments | installment																						3
Settlement receivable installment amount			$ 3,000,000	$ 3,000,000			$ 6,000,000
Equity Commonwealth
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Total rent due										$ 9,200,000