Noncontrolling Interest in Consolidated Subsidiary (Details) (SIR, USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended
	Mar. 12, 2012	Dec. 31, 2013	Dec. 31, 2012
Noncontrolling Interest in Consolidated Subsidiary
Less than wholly owned subsidiary, common shares owned		22,000,000	22,000,000
Equity method investments, ownership percentage		44.20%
Less than wholly owned subsidiary, parent ownership interest percentage			56.00%
Ownership percentage prior to initial public offering	100.00%	100.00%
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)		55.99%	70.40%
Effect on the Equity Attributable to CommonWealth REIT Common Shareholders		32	(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)