Indebtedness	6 Months Ended
Jun. 30, 2017
Debt Disclosure [Abstract]
Indebtedness	Indebtedness Our principal debt obligations at June 30, 2017 were: (1) outstanding borrowings under our $1,000,000 unsecured revolving credit facility; (2) six public issuances of senior unsecured notes, including: (a) $400,000 principal amount at an annual interest rate of 3.25% due 2019, (b) $200,000 principal amount at an annual interest rate of 6.75% due 2020, (c) $300,000 principal amount at an annual interest rate of 6.75% due 2021, (d) $250,000 principal amount at an annual interest rate of 4.75% due 2024, (e) $350,000 principal amount at an annual interest rate of 5.625% due 2042 and (f) $250,000 principal amount at an annual interest rate of 6.25% due 2046; (3) our $350,000 principal amount unsecured term loan due 2020; (4) our $200,000 principal amount unsecured term loan due 2022; and (5) $807,775 aggregate principal amount of mortgages (excluding premiums, discounts and net debt issuance costs) secured by 24 of our properties (25 buildings) with maturity dates between 2018 and 2043. The 24 mortgaged properties (25 buildings) had a carrying value (before accumulated depreciation) of $1,232,046 at June 30, 2017. We also had two properties subject to capital leases with lease obligations totaling $11,022 at June 30, 2017; these two properties had a carrying value (before accumulated depreciation) of $36,173 at June 30, 2017, and the capital leases expire in 2026. In April 2017, we prepaid a mortgage note secured by 17 of our properties with an outstanding principal balance of approximately $277,837 plus an aggregate premium of $5,449 plus accrued interest, a maturity date in September 2019 and an annual interest rate of 6.71%. In May 2017, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $10,579, a maturity date in August 2017 and an annual interest rate of 6.15%. In June 2017, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $8,807, a maturity date in August 2037 and an annual interest rate of 5.95%. We recorded loss on early extinguishment of debt of $7,353 for the three and six months ended June 30, 2017 related to these prepayments. We have a $1,000,000 revolving credit facility that is available for general business purposes, including acquisitions. Our revolving credit facility provides that we can borrow, repay and re-borrow funds available under our revolving credit facility until maturity, and no principal repayment is due until maturity. Our revolving credit facility required annual interest to be paid on borrowings at LIBOR plus a premium, which was 130 basis points as of June 30, 2017, plus a facility fee of 30 basis points per annum (as of June 30, 2017) on the total amount of lending commitments. In August 2017, we amended the agreement governing our revolving credit facility. As a result of the amendment, the interest rate payable on borrowings under the facility is reduced from LIBOR plus a premium of 130 basis points per annum (as of June 30, 2017) to LIBOR plus a premium of 120 basis points per annum, and the facility fee is reduced from 30 basis points per annum (as of June 30, 2017) to 25 basis points per annum on the total amount of lending commitments under the facility. The interest rate premium and facility fee are each subject to adjustment based upon changes to our credit ratings. Also as a result of the amendment, the stated maturity date of the facility was extended from January 15, 2018 to January 15, 2022, and, subject to the payment of an extension fee and meeting other conditions, we have the option to extend the maturity date of the facility for an additional year. The facility also includes a feature pursuant to which in certain circumstances maximum borrowings under the facility may be increased to up to $2,000,000. As of June 30, 2017, the annual interest rate payable on borrowings under our revolving credit facility was 2.5%. The weighted average annual interest rates for borrowings under our revolving credit facility were 2.3% and 1.7% for the three months ended June 30, 2017 and 2016, respectively, and 2.2% and 1.7% for the six months ended June 30, 2017 and 2016, respectively. As of June 30, 2017, we had $434,000 outstanding and $566,000 available for borrowing, and as of August 2, 2017, we had $415,000 outstanding and $585,000 available for borrowing under our revolving credit facility. We incurred interest expense and other associated costs related to our revolving credit facility of $2,662 and $3,454 for the three months ended June 30, 2017 and 2016, respectively, and $5,026 and $7,136 for the six months ended June 30, 2017 and 2016, respectively. We have a $350,000 term loan, which we borrowed in 2014. This term loan matures in January 2020, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at LIBOR plus a premium of 140 basis points that is subject to adjustment based upon changes to our credit ratings. At June 30, 2017, the annual interest rate payable on amounts outstanding under this term loan was 2.5%. The weighted average annual interest rate for amounts outstanding under this term loan was 2.5% and 1.9% for the three months ended June 30, 2017 and 2016, respectively, and 2.3% and 1.9% for the six months ended June 30, 2017 and 2016, respectively. We incurred interest expense and other associated costs related to this term loan of $2,140 and $1,635 for the three months ended June 30, 2017 and 2016, respectively, and $4,056 and $3,249 for the six months ended June 30, 2017 and 2016, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $700,000 in certain circumstances. We also have a $200,000 term loan, which we borrowed in 2015. This term loan matures in September 2022, and is prepayable without penalty at any time beginning in September 2017. At June 30, 2017, the annual interest rate payable on amounts outstanding under this term loan was 3.0%. The weighted average annual interest rate for amounts outstanding under this term loan was 2.9% and 2.3% for the three months ended June 30, 2017 and 2016, respectively, and 2.8% and 2.3% for the six months ended June 30, 2017 and 2016, respectively. We incurred interest expense and other associated costs related to this term loan of $1,427 and $1,133 for the three months ended June 30, 2017 and 2016, respectively, and $2,725 and $2,260 for the six months ended June 30, 2017 and 2016, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $400,000 in certain circumstances. In August 2017, we amended the agreement governing this term loan. As a result of the amendment, the interest rate payable is reduced from LIBOR plus a premium of 180 basis points per annum (as of June 30, 2017) to LIBOR plus a premium of 135 basis points per annum, subject to adjustment based upon changes to our credit ratings. Our revolving credit facility and term loan agreements and our senior unsecured notes indentures and their 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 revolving credit facility and term loan agreements, a change of control of us, as defined, which includes The RMR Group LLC, or RMR LLC, ceasing to act as our business and property manager. Our revolving credit facility and term loan agreements and our senior unsecured notes indentures and their supplements also contain a number of covenants, including covenants that restrict our ability to incur debts, and generally require us to maintain certain financial ratios, and our revolving credit facility and term loan agreements 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 revolving credit facility and term loan agreements and our senior unsecured notes indentures and their supplements at June 30, 2017.

Indebtedness

Our principal debt obligations at June 30, 2017 were: (1) outstanding borrowings under our $1,000,000 unsecured revolving credit facility; (2) six public issuances of senior unsecured notes, including: (a) $400,000 principal amount at an annual interest rate of 3.25% due 2019, (b) $200,000 principal amount at an annual interest rate of 6.75% due 2020, (c) $300,000 principal amount at an annual interest rate of 6.75% due 2021, (d) $250,000 principal amount at an annual interest rate of 4.75% due 2024, (e) $350,000 principal amount at an annual interest rate of 5.625% due 2042 and (f) $250,000 principal amount at an annual interest rate of 6.25% due 2046; (3) our $350,000 principal amount unsecured term loan due 2020; (4) our $200,000 principal amount unsecured term loan due 2022; and (5) $807,775 aggregate principal amount of mortgages (excluding premiums, discounts and net debt issuance costs) secured by 24 of our properties (25 buildings) with maturity dates between 2018 and 2043. The 24 mortgaged properties (25 buildings) had a carrying value (before accumulated depreciation) of $1,232,046 at June 30, 2017. We also had two properties subject to capital leases with lease obligations totaling $11,022 at June 30, 2017; these two properties had a carrying value (before accumulated depreciation) of $36,173 at June 30, 2017, and the capital leases expire in 2026.

In April 2017, we prepaid a mortgage note secured by 17 of our properties with an outstanding principal balance of approximately $277,837 plus an aggregate premium of $5,449 plus accrued interest, a maturity date in September 2019 and an annual interest rate of 6.71%. In May 2017, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $10,579, a maturity date in August 2017 and an annual interest rate of 6.15%. In June 2017, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $8,807, a maturity date in August 2037 and an annual interest rate of 5.95%. We recorded loss on early extinguishment of debt of $7,353 for the three and six months ended June 30, 2017 related to these prepayments.

We have a $1,000,000 revolving credit facility that is available for general business purposes, including acquisitions. Our revolving credit facility provides that we can borrow, repay and re-borrow funds available under our revolving credit facility until maturity, and no principal repayment is due until maturity. Our revolving credit facility required annual interest to be paid on borrowings at LIBOR plus a premium, which was 130 basis points as of June 30, 2017, plus a facility fee of 30 basis points per annum (as of June 30, 2017) on the total amount of lending commitments.

In August 2017, we amended the agreement governing our revolving credit facility. As a result of the amendment, the interest rate payable on borrowings under the facility is reduced from LIBOR plus a premium of 130 basis points per annum (as of June 30, 2017) to LIBOR plus a premium of 120 basis points per annum, and the facility fee is reduced from 30 basis points per annum (as of June 30, 2017) to 25 basis points per annum on the total amount of lending commitments under the facility. The interest rate premium and facility fee are each subject to adjustment based upon changes to our credit ratings. Also as a result of the amendment, the stated maturity date of the facility was extended from January 15, 2018 to January 15, 2022, and, subject to the payment of an extension fee and meeting other conditions, we have the option to extend the maturity date of the facility for an additional year. The facility also includes a feature pursuant to which in certain circumstances maximum borrowings under the facility may be increased to up to $2,000,000.

As of June 30, 2017, the annual interest rate payable on borrowings under our revolving credit facility was 2.5%. The weighted average annual interest rates for borrowings under our revolving credit facility were 2.3% and 1.7% for the three months ended June 30, 2017 and 2016, respectively, and 2.2% and 1.7% for the six months ended June 30, 2017 and 2016, respectively. As of June 30, 2017, we had $434,000 outstanding and $566,000 available for borrowing, and as of August 2, 2017, we had $415,000 outstanding and $585,000 available for borrowing under our revolving credit facility. We incurred interest expense and other associated costs related to our revolving credit facility of $2,662 and $3,454 for the three months ended June 30, 2017 and 2016, respectively, and $5,026 and $7,136 for the six months ended June 30, 2017 and 2016, respectively.

We have a $350,000 term loan, which we borrowed in 2014. This term loan matures in January 2020, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at LIBOR plus a premium of 140 basis points that is subject to adjustment based upon changes to our credit ratings. At June 30, 2017, the annual interest rate payable on amounts outstanding under this term loan was 2.5%. The weighted average annual interest rate for amounts outstanding under this term loan was 2.5% and 1.9% for the three months ended June 30, 2017 and 2016, respectively, and 2.3% and 1.9% for the six months ended June 30, 2017 and 2016, respectively. We incurred interest expense and other associated costs related to this term loan of $2,140 and $1,635 for the three months ended June 30, 2017 and 2016, respectively, and $4,056 and $3,249 for the six months ended June 30, 2017 and 2016, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $700,000 in certain circumstances.

We also have a $200,000 term loan, which we borrowed in 2015. This term loan matures in September 2022, and is prepayable without penalty at any time beginning in September 2017. At June 30, 2017, the annual interest rate payable on amounts outstanding under this term loan was 3.0%. The weighted average annual interest rate for amounts outstanding under this term loan was 2.9% and 2.3% for the three months ended June 30, 2017 and 2016, respectively, and 2.8% and 2.3% for the six months ended June 30, 2017 and 2016, respectively. We incurred interest expense and other associated costs related to this term loan of $1,427 and $1,133 for the three months ended June 30, 2017 and 2016, respectively, and $2,725 and $2,260 for the six months ended June 30, 2017 and 2016, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $400,000 in certain circumstances. In August 2017, we amended the agreement governing this term loan. As a result of the amendment, the interest rate payable is reduced from LIBOR plus a premium of 180 basis points per annum (as of June 30, 2017) to LIBOR plus a premium of 135 basis points per annum, subject to adjustment based upon changes to our credit ratings.

Our revolving credit facility and term loan agreements and our senior unsecured notes indentures and their 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 revolving credit facility and term loan agreements, a change of control of us, as defined, which includes The RMR Group LLC, or RMR LLC, ceasing to act as our business and property manager. Our revolving credit facility and term loan agreements and our senior unsecured notes indentures and their supplements also contain a number of covenants, including covenants that restrict our ability to incur debts, and generally require us to maintain certain financial ratios, and our revolving credit facility and term loan agreements 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 revolving credit facility and term loan agreements and our senior unsecured notes indentures and their supplements at June 30, 2017.