Summary of Significant Accounting Policies - Risk Management and Use of Financial Instruments (Details)	12 Months Ended
Dec. 31, 2017 item
Accounting Policies [Abstract]
Number of main components of economic risk	3

Represents the number of components of economic risk.

No definition available.

Name:	exr_EconomicRiskNumberOfComponents
Namespace Prefix:	exr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration