Share-Based Compensation - Narrative (Details) $ / shares in Units, $ in Millions								12 Months Ended
	Jun. 13, 2023 USD ($) trustee $ / shares shares	Jan. 26, 2023 $ / shares shares	Jun. 21, 2022 USD ($) trustee $ / shares shares	Jan. 26, 2022 $ / shares shares	Jun. 23, 2021 USD ($) trustee $ / shares shares	Jan. 25, 2021 $ / shares shares	Jun. 16, 2015	Dec. 31, 2027 shares	Dec. 31, 2026 shares	Dec. 31, 2025 shares	Dec. 31, 2024 shares	Dec. 31, 2023 USD ($) tranche $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 shares	Jun. 20, 2019 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	1,650,000
Common shares available for issuance (in shares)												2,073,350
Number of independent trustees | trustee	6		6		6
Common stock issued (in shares)												106,847,438	109,428,252
Compensation expense, accelerated vesting due to a staffing reduction | $												$ 5.2	$ 0.4	$ 3.5
Equity Commonwealth 2015 Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)																2,500,000
Share-based compensation arrangement, plan term							10 years
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense | $												$ 16.0	$ 11.9	$ 15.4
Restricted shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		132,794	18,020	92,573	18,505	122,466
Granted (in dollars per share) | $ / shares		$ 25.61		$ 25.50		$ 28.25
Common stock issued (in shares)														523,662
Restricted shares | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)	28,865
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period												3 years
Number of tranches | tranche												2
Vested (in shares)												274,739	382,993	523,662
Common stock issued (in shares)												274,739
RSUs | Share-based Payment Arrangement, Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage												50.00%
RSUs | Share-based Payment Arrangement, Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period												1 year
Vesting percentage												50.00%
RSUs, target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		269,609		187,951		248,646
RSUs, maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)		672,000		468,468		619,750
LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity award, conversion basis												1
Number of equity awards granted (in shares)	5,773
Time-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			3,604	29,071	3,701
Market-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)														81,434
Conversion of equity instruments (in shares)														81,434
Market-Based LTIP Units, target
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)				59,024
Market-Based LTIP Units At Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)				147,117
OP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion of equity instruments (in shares)														81,434
Restricted Shares and Time-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)	5,773											167,432	143,268	144,672
Granted (in dollars per share) | $ / shares												$ 24.61	$ 25.84	$ 28.06
Share based compensation amount | $	$ 0.1		$ 0.1		$ 0.1
Price per share (in dollars per share) | $ / shares	$ 20.79		$ 27.75		$ 27.02
Vested (in shares)												195,521	125,958	188,990
Unvested (in shares)												330,591	358,680	341,370	385,688
Estimated future compensation expense for unvested shares | $												$ 4.3
Weighted average period over which compensation expense will be recorded												2 years 3 months 18 days
Restricted Shares and Time-Based LTIP Units | Independent Trustee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)			3,604		3,701
Restricted Shares and Time-Based LTIP Units | Scenario, Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)								53,503	73,427	90,554	113,107
RSUs and Market-Based LTIP Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of equity awards granted (in shares)												672,000	615,585	619,750
Granted (in dollars per share) | $ / shares												$ 14.65	$ 14.09	$ 15.19
Vested (in shares)												350,484	382,993	605,096
Unvested (in shares)												2,038,776	1,853,472	1,979,572	1,964,918
Estimated future compensation expense for unvested shares | $												$ 9.7
Weighted average period over which compensation expense will be recorded												2 years 2 months 12 days
Common Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock issued (in shares)													382,993
Maximum | Equity Commonwealth 2015 Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares available for issuance (in shares)												7,400,000