Noncontrolling Interest in Consolidated Subsidiary (Details) (SIR, USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Noncontrolling Interest in Consolidated Subsidiary
Less than wholly owned subsidiary, common shares owned	22,000,000
Less than wholly owned subsidiary, parent ownership interest percentage	56.00%
Percentage of ownership interest in subsidiary (as a percent)	100.00%
Less than wholly owned subsidiary, noncontrolling interest ownership interest percentage	44.00%
Effect on the Equity Attributable to CommonWealth REIT Common Shareholders	$ 24,817
IPO
Noncontrolling Interest in Consolidated Subsidiary
CommonWealth REIT Ownership Percentage of SIR After Transaction (as a percent)	70.50%
Effect on the Equity Attributable to CommonWealth REIT Common Shareholders	23,857
Equity compensation plan
Noncontrolling Interest in Consolidated Subsidiary
CommonWealth REIT Ownership Percentage of SIR After Transaction (as a percent)	70.40%
Effect on the Equity Attributable to CommonWealth REIT Common Shareholders	253
Common Shares
Noncontrolling Interest in Consolidated Subsidiary
CommonWealth REIT Ownership Percentage of SIR After Transaction (as a percent)	56.00%
Effect on the Equity Attributable to CommonWealth REIT Common Shareholders	$ 707