Related Person Transactions - Narrative (Details) (USD $)	0 Months Ended		12 Months Ended			0 Months Ended	1 Months Ended	6 Months Ended	3 Months Ended	9 Months Ended	0 Months Ended		3 Months Ended		0 Months Ended
	Jul. 31, 2014 option	Jun. 01, 2014 option	Dec. 31, 2014 sqft	Dec. 31, 2013 sqft	Dec. 31, 2012	Aug. 05, 2014	Jan. 31, 2013	Jun. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Mar. 15, 2013	May 09, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 19, 2013 installment	Jul. 09, 2014	Jul. 02, 2013	Mar. 12, 2012	Feb. 16, 2012 building property sqft	Mar. 01, 2014 property	Feb. 28, 2015	May 23, 2014 trustee	May 08, 2014	Jul. 03, 2013	Jul. 01, 2013	Mar. 31, 2013
Related person transactions
Lease term	7 months	1 year
Number of renewal options	1	1
Renewal option term	3 months	3 months
Lease payment for initial term	$ 200,000	$ 200,000
Amount of reimbursements payable to related party			0	9,385,000					0
Number of individuals elected to serve on Board of Trustees																						7
Property square feet			42,919,000	42,915,000					42,919,000
Number of shares granted under the Award Plan (in shares)			4,000	83,450	83,617
Equity Group Investments
Related person transactions
License agreement term		1 year
License fee		200,000
Number of parking spaces		1
Related/Corvex
Related person transactions
Amount of reimbursements payable to related party	33,500,000
Reimbursement amount paid			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
Ownership percentage						5.70%
Number of individuals elected to serve on Board of Trustees																						7
Related Fund Management LLC
Related person transactions
Ownership percentage						2.80%
RMR
Related person transactions
Lease term			3 years
Number of agreements			3						3
Number of former managing trustees			1
Number of former independent trustees			2
Annual business management fee as a percentage of aggregate cost of properties acquired in excess of $250,000				0.50%	0.50%
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%	1.00%
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%	15.00%
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%	0.70%
Business management fees								31,700,000
Shares issued (in shares)								68,206
Incentive fees paid				0	0
Property management fees as a percentage of gross collected rents			3.00%						3.00%
Construction supervision fees as a percentage of construction costs			5.00%						5.00%
Payments to MacArthur Cook Fund Management Limited					1,800,000		200,000
Number of regional offices leased			11
Rental income earned				800,000	600,000			400,000
Property square feet			36,500						36,500
Legal fees and costs				30,400,000				5,400,000
Number of properties																				2
Total rent due			1,100,000						1,100,000
RMR Australia
Related person transactions
Property management fees as a percentage of gross collected rents			3.00%						3.00%
Compensation for construction supervision services as a percentage of difference between 5.0% of constructions costs and any amounts paid to third parties for construction management and/or supervision			50.00%						50.00%
Construction supervision fees as a percentage of construction costs			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%
Compensation for property management services as a percentage of difference between 3.0% 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%
Business and property management fees				1,600,000				900,000
RMR and RMR Australia
Related person transactions
Monthly management fees									1,200,000
Pro-rata portion of the current year’s incentive fee under the existing business management agreement										15,300,000
GOV
Related person transactions
Amount of reimbursements payable to related party				300,000
Prior ownership percentage			100.00%
Shares sold (in 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 person transactions
Ownership Percentage																0.00%							12.50%
Equity investments, number of shares sold (in shares)												20,000
Equity investments, sales proceeds												5,800,000
Amount invested in equity investee												5,200,000
Number of other companies which are shareholders of related party												4
Coverage amount for property insurance								500,000,000						500,000,000
Premium for property insurance					6,600,000			6,000,000
Equity Commonwealth and REIT Management and Research LLC
Related person transactions
Settlement receivable			12,000,000						12,000,000
Number of settlement receivable installments			3
Settlement receivable installment amount									3,000,000				3,000,000	6,000,000
Equity Commonwealth
Related person transactions
Total rent due			9,200,000						9,200,000
Minimum
Related person transactions
Price per common share during one year period after date (in dollars per share)	$ 26.00
Price per common share during first and second year after date (in dollars per share)	$ 26.00
Maximum
Related person transactions
Ownership Percentage			50.00%						50.00%
Maximum | RMR
Related person transactions
Threshold amount of real estate investments for payment of base management fee				250,000,000	250,000,000
Amended Agreement | RMR
Related person transactions
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 market capitalization exceeding specified amount															0.50%
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 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%							10.00%
Incentive management fee payable (as a percent)															12.00%
Measurement period of incentive management fee															3 years
Percentage of common shares issued in payment of an incentive management fee vested on the date of issuance															33.00%
Percentage of common shares issued in payment of an incentive management fee vesting after date of issuance															67.00%
Number of equal annual installments for vesting of common shares															2
Amended Agreement | Maximum | RMR
Related person transactions
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
SIR
Related person transactions
Amount of reimbursements payable to related party																										600,000
Number of former independent trustees			1
Property square feet																			21,400,000
Prior ownership percentage			100.00%															100.00%
Shares sold (in shares)																	10,500,000	9,200,000
Less than wholly owned subsidiary, common shares owned (in shares)				22,000,000	22,000,000			22,000,000						22,000,000		22,000,000									22,000,000	22,000,000
Ownership Percentage				44.20%				36.70%						36.70%		0.00%								44.20%
Proceeds from IPO																		180,800,000
Repayment of promissory note																			400,000,000
Common shares consideration received on transfer of properties (in shares)																			22,000,000
Number of properties sold or transferred or agreed to be sold or transferred																			79
Number of separate buildings or land parcels sold or transferred or agreed to be sold or transferred																			251
Equity investments, number of shares sold (in shares)																22,000,000
Share price (in dollars per share)																$ 32.04
Equity investments, sales proceeds																704,800,000
SIR | RMR
Related person transactions
Business management fees				43,300,000	43,600,000
Property management and construction supervision fees				32,500,000	33,700,000			13,800,000
Pro rata share of related party's costs of providing internal audit function				300,000	400,000
Parent Company
Related person transactions
Number of properties																				1
Parent Company | RMR
Related person transactions
Number of properties acquired or agreed to be acquired																				1
Restricted shares
Related person transactions
Number of shares granted under the Award Plan (in shares)			713,727
Number of anniversaries of the grant date for vesting of restricted shares			4 years
Restricted shares | RMR
Related person transactions
Number of shares granted under the Award Plan (in shares)				73,450	71,617
Aggregate value of restricted shares granted				1,700,000	1,100,000
Number of anniversaries of the grant date for vesting of restricted shares			4 years
Vesting Rights Percentage			20.00%
Vesting Rights Percentage on Each of Next Four Anniversaries			20.00%
Scenario, Forecast | RMR and RMR Australia
Related person transactions
Monthly management fees																					$ 100,000
2014 | Amended Agreement | RMR
Related person transactions
Measurement period of incentive management fee															1 year
2015 | Amended Agreement | RMR
Related person transactions
Measurement period of incentive management fee															2 years