Equity Investment in Select Income REIT - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity in earnings of investees	$ 9,712	$ 2,739
Investment at carrying value	465,131		$ 467,499
Cash distributions from SIR	$ 10,289	2,611
SIR
Schedule of Equity Method Investments [Line Items]
Equity investments, common shares owned (in shares)	24,918,421
Percentage of outstanding shares owned	27.80%
Equity in earnings of investees	$ 10,289	2,611
Equity in unrealized gain (loss) of investees	51	4,492
The amount of investment in exceed the underlying equity of the investee	122,579		$ 87,137
Accretion in equity of earnings	1,044	736
Investment at carrying value	465,131
Equity Investments, market value	485,411
Cash distributions from SIR	$ 12,708	$ 12,708