Investment in Direct Financing Lease - Narrative (Details)	Dec. 31, 2017
Capital Leases, Net Investment in Direct Financing Leases [Abstract]
Percentage of term exceeding useful life of one office tower	75.00%

Represents the percentage of the term exceeding the useful life of one office tower.

No definition available.

Name:	eqc_PercentageOfTermExceedingUsefulLifeOfOneOfficeTower
Namespace Prefix:	eqc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant