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 USD ($) option	Jun. 16, 2015 shares	May. 04, 2015 USD ($)	Mar. 01, 2015 USD ($)	Jan. 28, 2015 shares	Dec. 31, 2014 USD ($) ft² property	Sep. 30, 2014 USD ($) trustee	Sep. 29, 2014 trustee agreement	Jul. 31, 2014 USD ($) option $ / shares	Jul. 09, 2014 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) ft² shares	Jun. 01, 2014 USD ($) option parking_space	May. 09, 2014 USD ($) company shares	Mar. 01, 2014 property	Dec. 19, 2013 USD ($)	Jul. 02, 2013 shares	Mar. 15, 2013 USD ($) shares	Mar. 12, 2012 shares	Feb. 29, 2012	Feb. 16, 2012 USD ($) ft² building property shares	Jan. 31, 2016 USD ($)	Aug. 31, 2015 USD ($)	Jan. 31, 2013 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 ($)	Dec. 31, 2015 shares	Dec. 31, 2014 USD ($) ft² property trustee installment shares	Dec. 31, 2013 USD ($) shares	Dec. 31, 2009	May. 31, 2014 USD ($)	May. 08, 2014	Mar. 31, 2014 ft²	Jul. 01, 2013 shares	Mar. 31, 2013 USD ($)
Related Party Transaction [Line Items]
Property square feet | ft²						5,600,000																								5,600,000					5,641,450
Number of equity awards granted (in shares) | shares																														4,000	83,450
Number of properties | property						31																								31
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%
Twenty Percent on Grant Date
Related Party Transaction [Line Items]
Vesting percent																													20.00%
Twenty Percent in Year One
Related Party Transaction [Line Items]
Vesting percent																													20.00%
Twenty Percent in Year Two
Related Party Transaction [Line Items]
Vesting percent																													20.00%
Twenty Percent in Year Three
Related Party Transaction [Line Items]
Vesting percent																													20.00%
Twenty Percent in Year Four
Related Party Transaction [Line Items]
Vesting percent																													20.00%
SIR
Related Party Transaction [Line Items]
Amount of reimbursements payable to related party																																					$ 600,000
Number of former independent trustees | trustee																														1
Property square feet | ft²																				21,400,000
Prior ownership percentage																		100.00%	100.00%
Shares sold (in shares) | shares																10,500,000		9,200,000						9,200,000
Less than wholly owned subsidiary, common shares owned (in shares) | shares										22,000,000	22,000,000														22,000,000		22,000,000									22,000,000
Ownership Percentage										0.00%	36.70%					44.20%									36.70%		36.70%
Proceeds from IPO																								$ 180,800,000
Repayment of promissory note																				$ 400,000,000
Common shares consideration received on transfer of properties (in shares) | shares																				22,000,000
Number of properties sold or transferred or agreed to be sold or transferred | property																				79
Number of separate buildings or land parcels sold or transferred or agreed to be sold or transferred | building																				251
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
Parent Company
Related Party Transaction [Line Items]
Number of properties | property														1
Amended Agreement
Related Party Transaction [Line Items]
Base business management fee payable in common shares (as a percent)											10.00%														10.00%		10.00%
Restricted shares
Related Party Transaction [Line Items]
Number of equity awards granted (in shares) | shares		34,587			126,319																								160,906	713,727
Number of anniversaries of the grant date for vesting of restricted shares		1 year			4 years
Minimum
Related Party Transaction [Line Items]
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
Maximum
Related Party Transaction [Line Items]
Ownership Percentage																													50.00%
Equity Group Investments and associated entities
Related Party Transaction [Line Items]
Initial term	$ 500,000								$ 200,000			$ 200,000
Lease term	5 years								7 months			1 year
Number of renewal options | option	1								1			1
Renewal option term	5 years								3 months			3 months
Lease payment	$ 800,000		$ 200,000
License agreement term												1 year
License fee			$ 100,000									$ 200,000
Number of parking spaces | parking_space												1
Equity Group Investments and associated entities | Subsequent event
Related Party Transaction [Line Items]
Lease payment																					$ 100,000
License fee																					$ 100,000
Related/Corvex
Related Party Transaction [Line Items]
Amount of reimbursements payable to related party									$ 33,500,000
Reimbursement amount paid																						$ 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
RMR
Related Party Transaction [Line Items]
Lease term																									3 years
Number of agreements | agreement								3
Number of former managing trustees | trustee								1
Number of former independent trustees | trustee							2
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%
Business management fees																											$ 31,700,000
Shares issued (in shares) | shares																											68,206
Base management fee payable as a percentage of average historical cost of real estate investments, located in the United States, Puerto Rico and Canada																															0.70%
Annual business management fee as a percentage of aggregate cost of properties acquired in excess of $250,000																															0.50%
Incentive fee as a percentage of the product of weighted average common shares outstanding on a fully diluted basis and the excess, if any of the Normalized FFO per share for current fiscal year over the preceding fiscal year																															15.00%
Incentive fees paid																															$ 0
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
Payments to MacArthur Cook Fund Management Limited																							$ 200,000
Number of regional offices leased | office																									11
Rental income earned																											$ 400,000				800,000
Property square feet | ft²											36,500														36,500		36,500
Legal fees and costs																											$ 5,400,000				30,400,000
Number of properties | property														2
Total rent due						$ 1,100,000																								$ 1,100,000
RMR | SIR
Related Party Transaction [Line Items]
Business management fees																															43,300,000
Property management and construction supervision fees																															32,500,000
Pro rata share of related party's costs of providing internal audit function																															$ 300,000
RMR | Parent Company
Related Party Transaction [Line Items]
Number of properties acquired or agreed to be acquired | property														1
RMR | Amended Agreement
Related Party Transaction [Line Items]
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 (as a percent)															90.00%
Base business management fee payable in common shares (as a percent)															10.00%
Minimum ownership of outstanding common shares of related party required to engage the same manager or common managing trustees															10.00%
Related Party Transaction Percentage for Limitation and Adjustments of Incentive Management Fee Payable															12.00%
Measurement period of incentive management fee															3 years
RMR | Restricted shares
Related Party Transaction [Line Items]
Number of equity awards granted (in shares) | shares																															73,450
Aggregate value of restricted shares granted																															$ 1,700,000
Number of anniversaries of the grant date for vesting of restricted shares																									4 years
RMR | 2014 | Amended Agreement
Related Party Transaction [Line Items]
Measurement period of incentive management fee																														1 year
RMR | 2015 | Amended Agreement
Related Party Transaction [Line Items]
Measurement period of incentive management fee																													2 years
RMR | Maximum
Related Party Transaction [Line Items]
Threshold amount of real estate investments for payment of base management fee																															250,000,000
RMR | Maximum | Amended Agreement
Related Party Transaction [Line Items]
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
Related Party Transaction [Line Items]
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%
Compensation for business management and real estate investment services as a percentage of average historical cost of CWHAT's real estate investments											0.50%														0.50%		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%														50.00%		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%														50.00%		50.00%
Business and property management fees																											$ 900,000				1,600,000
RMR and RMR Australia
Related Party Transaction [Line Items]
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
Related Party Transaction [Line Items]
Amount of reimbursements payable to related party																															$ 300,000
Prior ownership percentage																																100.00%
Shares sold (in shares) | shares																	9,950,000
Net proceeds received on issuance or sale of common shares																	$ 239,600,000
Realized gain on sale of common shares																	$ 66,300,000
AIC
Related Party Transaction [Line Items]
Ownership Percentage													0.00%																					12.50%
Equity investments, number of shares sold (in shares) | shares													20,000
Equity investments, proceeds from sale													$ 5,800,000
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
Related Party Transaction [Line Items]
Settlement receivable						12,000,000																								$ 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
Related Party Transaction [Line Items]
Total rent due						$ 9,200,000																								$ 9,200,000