Equity Investment in Select Income REIT - Balance Sheet Information (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Real estate properties, net	$ 1,598,642	$ 1,591,956
Acquired real estate leases, net	99,953	124,848
Cash and cash equivalents	551,707	29,941	$ 13,749	$ 8,785
Rents receivable, net	47,461	48,458
Other assets, net	89,484	68,005
Total assets	3,536,967	2,385,066
Unsecured revolving credit facility	565,000	160,000
Unsecured term loan, net	547,682	547,171
Senior unsecured notes, net	943,543	646,844
Mortgage notes payable, net	26,561	27,837
Assumed real estate lease obligations, net	8,832	10,626
Other liabilities	63,525	54,019
Shareholders' equity	1,377,527	935,004
Total liabilities and shareholders’ equity	3,536,967	2,385,066
SIR | SIR
Schedule of Equity Method Investments [Line Items]
Real estate properties, net	3,922,568	3,899,792
Acquired real estate leases, net	493,780	506,298
Properties held for sale	5,829	0
Cash and cash equivalents	18,155	22,127
Rents receivable, net	122,292	124,089
Other assets, net	114,771	87,376
Total assets	4,677,395	4,639,682
Unsecured revolving credit facility	102,000	327,000
Unsecured term loan, net	348,746	348,373
Senior unsecured notes, net	1,776,087	1,430,300
Mortgage notes payable, net	227,772	245,643
Assumed real estate lease obligations, net	70,989	77,622
Other liabilities	129,502	136,782
Shareholders' equity	2,022,299	2,073,962
Total liabilities and shareholders’ equity	$ 4,677,395	$ 4,639,682