Equity Investment in Select Income REIT - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity in earnings of investees	$ 9,484	$ 8,668	$ 20,804	$ 28,002
Investment at carrying value	$ 475,265		475,265		$ 487,708
Cash distributions from SIR			$ 18,062	25,676
SIR
Schedule of Equity Method Investments [Line Items]
Equity investments, common shares owned (in shares)	24,918,421		24,918,421
Percentage of outstanding shares owned	27.80%		27.80%
Equity in earnings of investees	$ 9,453	8,655	$ 20,271	27,895
Equity in unrealized gain (loss) of investees	1,236	3,192	5,339	10,248
The amount of investment in exceed the underlying equity of the investee	87,976		87,976
Accretion in equity of earnings	736	740	2,209	2,219
Investment at carrying value	475,265		475,265
Equity Investments, market value	583,589		583,589
Cash distributions from SIR	$ 12,708	$ 12,708	$ 38,125	$ 37,627