Indebtedness	3 Months Ended
Mar. 31, 2017
Debt Disclosure [Abstract]
Indebtedness	Indebtedness Our principal debt obligations at March 31, 2017 were: (1) $160,000 of outstanding borrowings under our $750,000 unsecured revolving credit facility; (2) an aggregate outstanding principal amount of $550,000 of unsecured term loans; (3) an aggregate outstanding principal amount of $660,000 of public issuances of senior unsecured notes; and (4) $27,129 aggregate principal amount of mortgage notes. Our $750,000 revolving credit facility, our $300,000 term loan and our $250,000 term loan are governed by a credit agreement with a syndicate of institutional lenders that includes a number of features common to all of these credit arrangements. This credit agreement also includes a feature under which the maximum aggregate borrowing availability may be increased to up to $2,500,000 on a combined basis in certain circumstances. Our $750,000 revolving credit facility is available for general business purposes, including acquisitions. The maturity date of our revolving credit facility is January 31, 2019 and, subject to the payment of an extension fee and meeting other conditions, we have an option to extend the stated maturity date of our revolving credit facility by one year to January 31, 2020. We can borrow, repay and reborrow funds available under our revolving credit facility until maturity and no principal repayment is due until maturity. We are required to pay interest at a rate of LIBOR plus a premium, which was 125 basis points per annum at March 31, 2017, on borrowings under our revolving credit facility. We also pay a facility fee on the total amount of lending commitments under our revolving credit facility, which was 25 basis points per annum at March 31, 2017. Both the interest rate premium and the facility fee are subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate payable on borrowings under our revolving credit facility was 2.2% and the weighted average annual interest rate for borrowings under our revolving credit facility was 2.0% and 1.6%, respectively, for the three months ended March 31, 2017 and 2016. As of both March 31, 2017 and April 25, 2017, we had $160,000 outstanding under our revolving credit facility. Our $300,000 term loan, which matures on March 31, 2020, is prepayable without penalty at any time. We are required to pay interest at a rate of LIBOR plus a premium, which was 140 basis points per annum at March 31, 2017, on the amount outstanding under our $300,000 term loan. The interest rate premium is subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate for the amount outstanding under our $300,000 term loan was 2.4%. The weighted average annual interest rate under our $300,000 term loan was 2.2% and 1.8%, respectively, for the three months ended March 31, 2017 and 2016. Our $250,000 term loan, which matures on March 31, 2022, is prepayable without penalty at any time. We are required to pay interest at a rate of LIBOR plus a premium, which was 180 basis points per annum as of March 31, 2017, on the amount outstanding under our $250,000 term loan. The interest rate premium is subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate for the amount outstanding under our $250,000 term loan was 2.8%. The weighted average annual interest rate under our $250,000 term loan was 2.6% and 2.2%, respectively, for the three months ended March 31, 2017 and 2016. Our credit agreement and senior notes indenture and its supplements provide for acceleration of payment of all amounts due thereunder upon the occurrence and continuation of certain events of default, such as, in the case of our credit agreement, a change of control of us, which includes The RMR Group LLC, or RMR LLC, ceasing to act as our business and property manager. Our credit agreement and our senior notes indenture and its supplements also contain a number of covenants, including covenants that restrict our ability to incur debts, require us to maintain certain financial ratios and, in the case of our credit agreement, restrict our ability to make distributions under certain circumstances. We believe we were in compliance with the terms and conditions of the respective covenants under our credit agreement and senior notes indenture and its supplements at March 31, 2017. At March 31, 2017, three of our properties (three buildings) with an aggregate net book value of $51,600 are encumbered by three mortgages for an aggregate principal amount of $27,129. Our mortgage notes are non-recourse, subject to certain limited exceptions and do not contain any material financial covenants.

Indebtedness

Our principal debt obligations at March 31, 2017 were: (1) $160,000 of outstanding borrowings under our $750,000 unsecured revolving credit facility; (2) an aggregate outstanding principal amount of $550,000 of unsecured term loans; (3) an aggregate outstanding principal amount of $660,000 of public issuances of senior unsecured notes; and (4) $27,129 aggregate principal amount of mortgage notes.

Our $750,000 revolving credit facility, our $300,000 term loan and our $250,000 term loan are governed by a credit agreement with a syndicate of institutional lenders that includes a number of features common to all of these credit arrangements. This credit agreement also includes a feature under which the maximum aggregate borrowing availability may be increased to up to $2,500,000 on a combined basis in certain circumstances.

Our $750,000 revolving credit facility is available for general business purposes, including acquisitions. The maturity date of our revolving credit facility is January 31, 2019 and, subject to the payment of an extension fee and meeting other conditions, we have an option to extend the stated maturity date of our revolving credit facility by one year to January 31, 2020. We can borrow, repay and reborrow funds available under our revolving credit facility until maturity and no principal repayment is due until maturity. We are required to pay interest at a rate of LIBOR plus a premium, which was 125 basis points per annum at March 31, 2017, on borrowings under our revolving credit facility. We also pay a facility fee on the total amount of lending commitments under our revolving credit facility, which was 25 basis points per annum at March 31, 2017. Both the interest rate premium and the facility fee are subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate payable on borrowings under our revolving credit facility was 2.2% and the weighted average annual interest rate for borrowings under our revolving credit facility was 2.0% and 1.6%, respectively, for the three months ended March 31, 2017 and 2016. As of both March 31, 2017 and April 25, 2017, we had $160,000 outstanding under our revolving credit facility.

Our $300,000 term loan, which matures on March 31, 2020, is prepayable without penalty at any time. We are required to pay interest at a rate of LIBOR plus a premium, which was 140 basis points per annum at March 31, 2017, on the amount outstanding under our $300,000 term loan. The interest rate premium is subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate for the amount outstanding under our $300,000 term loan was 2.4%. The weighted average annual interest rate under our $300,000 term loan was 2.2% and 1.8%, respectively, for the three months ended March 31, 2017 and 2016.

Our $250,000 term loan, which matures on March 31, 2022, is prepayable without penalty at any time. We are required to pay interest at a rate of LIBOR plus a premium, which was 180 basis points per annum as of March 31, 2017, on the amount outstanding under our $250,000 term loan. The interest rate premium is subject to adjustment based upon changes to our credit ratings. As of March 31, 2017, the annual interest rate for the amount outstanding under our $250,000 term loan was 2.8%. The weighted average annual interest rate under our $250,000 term loan was 2.6% and 2.2%, respectively, for the three months ended March 31, 2017 and 2016.

Our credit agreement and senior notes indenture and its supplements provide for acceleration of payment of all amounts due thereunder upon the occurrence and continuation of certain events of default, such as, in the case of our credit agreement, a change of control of us, which includes The RMR Group LLC, or RMR LLC, ceasing to act as our business and property manager. Our credit agreement and our senior notes indenture and its supplements also contain a number of covenants, including covenants that restrict our ability to incur debts, require us to maintain certain financial ratios and, in the case of our credit agreement, restrict our ability to make distributions under certain circumstances. We believe we were in compliance with the terms and conditions of the respective covenants under our credit agreement and senior notes indenture and its supplements at March 31, 2017.

At March 31, 2017, three of our properties (three buildings) with an aggregate net book value of $51,600 are encumbered by three mortgages for an aggregate principal amount of $27,129. Our mortgage notes are non-recourse, subject to certain limited exceptions and do not contain any material financial covenants.