Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Summary of unaudited quarterly results of operations
Total revenues	$ 198,419	$ 198,283	$ 246,181	$ 242,653	$ 231,957	$ 222,632	$ 217,346	$ 209,970	$ 885,536	$ 881,905	$ 741,344
Net income (loss) available for CommonWealth REIT common shareholders	$ (16,452)	$ (227,466)	$ 7,738	$ 14,516	$ (163,935)	$ (122)	$ 2,242	$ 9,857	$ (221,664)	$ (151,958)	$ 62,999
Net income (loss) available for CommonWealth REIT common shareholders per share - basic and diluted (in dollars per share)	$ (0.14)	$ (1.92)	$ 0.07	$ 0.15	$ (1.96)		$ 0.03	$ 0.12	$ (1.97)	$ (1.81)	$ 0.81
Common distributions declared (in dollars per share)	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.50	$ 0.50	$ 0.50	$ 1.00	$ 1.75	$ 2.00