Indebtedness	9 Months Ended
Sep. 30, 2018
Debt Disclosure [Abstract]
Indebtedness	Indebtedness Our principal debt obligations at September 30, 2018 were: (1) outstanding borrowings under our $1,000,000 unsecured revolving credit facility; (2) seven 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) $500,000 principal amount at an annual interest rate of 4.75% due 2028, (f) $350,000 principal amount at an annual interest rate of 5.625% due 2042 and (g) $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) $735,605 aggregate principal amount of mortgages (excluding premiums, discounts and net debt issuance costs) secured by 13 of our properties (14 buildings) with maturity dates between 2019 and 2043. The 13 mortgaged properties (14 buildings) had a carrying value (before accumulated depreciation) of $1,033,303 at September 30, 2018. We also had two properties subject to capital leases with lease obligations totaling $10,053 at September 30, 2018; these two properties had a carrying value (before accumulated depreciation) of $36,257 at September 30, 2018, and the capital leases expire in 2026. In February 2018, we issued $500,000 of 4.75% senior unsecured notes due 2028, raising net proceeds of approximately $487,264 after underwriting discounts and expenses. We used the net proceeds of this offering to reduce amounts outstanding under our revolving credit facility. In January 2018, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $4,338, a maturity date in September 2043 and an annual interest rate of 4.4%. In July 2018, we prepaid, at par plus accrued interest, mortgage notes secured by 12 of our properties with an aggregate outstanding principal balance of approximately $90,602, maturity dates in October 2018 and a weighted average annual interest rate of 5.0%. In September 2018, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $6,325, a maturity date in January 2019 and an annual interest rate of 4.7%. We have a $1,000,000 revolving credit facility that is available for general business purposes, including acquisitions. The maturity date of our revolving credit facility is 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. 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 requires annual interest to be paid on borrowings at the rate of LIBOR plus a premium of 120 basis points, plus a facility fee of 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. As of September 30, 2018, the annual interest rate payable on borrowings under our revolving credit facility was 3.4%. The weighted average annual interest rates for borrowings under our revolving credit facility were 3.2% and 2.5% for the three months ended September 30, 2018 and 2017, respectively, and 2.9% and 2.3% for the nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, we had $195,000 outstanding and $805,000 available for borrowing, and as of November 5, 2018, we had $165,000 outstanding and $835,000 available for borrowing under our revolving credit facility. We incurred interest expense and other associated costs related to our revolving credit facility of $2,291 and $3,929 for the three months ended September 30, 2018 and 2017, respectively, and $7,872 and $9,888 for the nine months ended September 30, 2018 and 2017, respectively. 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. Our $350,000 term loan matures in January 2020, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at the rate of LIBOR plus a premium of 140 basis points, subject to adjustment based upon changes to our credit ratings. At September 30, 2018, the annual interest rate payable on amounts outstanding under this term loan was 3.5%. The weighted average annual interest rate for amounts outstanding under this term loan was 3.5% and 2.6% for the three months ended September 30, 2018 and 2017, respectively, and 3.3% and 2.4% for the nine months ended September 30, 2018 and 2017, respectively. We incurred interest expense and other associated costs related to this term loan of $3,263 and $2,491 for the three months ended September 30, 2018 and 2017, respectively, and $9,106 and $6,802 for the nine months ended September 30, 2018 and 2017, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $700,000 in certain circumstances. Our $200,000 term loan matures in September 2022, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at the rate of LIBOR plus a premium of 135 basis points, subject to adjustment based upon changes to our credit ratings. At September 30, 2018, the annual interest rate payable on amounts outstanding under this term loan was 3.6%. The weighted average annual interest rate for amounts outstanding under this term loan was 3.5% and 2.7% for the three months ended September 30, 2018 and 2017, respectively, and 3.3% and 2.7% for the nine months ended September 30, 2018 and 2017, respectively. We incurred interest expense and other associated costs related to this term loan of $1,842 and $1,476 for the three months ended September 30, 2018 and 2017, respectively, and $5,144 and $4,323 for the nine months ended September 30, 2018 and 2017, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $400,000 in certain circumstances. 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 September 30, 2018.

Indebtedness

Our principal debt obligations at September 30, 2018 were: (1) outstanding borrowings under our $1,000,000 unsecured revolving credit facility; (2) seven 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) $500,000 principal amount at an annual interest rate of 4.75% due 2028, (f) $350,000 principal amount at an annual interest rate of 5.625% due 2042 and (g) $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) $735,605 aggregate principal amount of mortgages (excluding premiums, discounts and net debt issuance costs) secured by 13 of our properties (14 buildings) with maturity dates between 2019 and 2043. The 13 mortgaged properties (14 buildings) had a carrying value (before accumulated depreciation) of $1,033,303 at September 30, 2018. We also had two properties subject to capital leases with lease obligations totaling $10,053 at September 30, 2018; these two properties had a carrying value (before accumulated depreciation) of $36,257 at September 30, 2018, and the capital leases expire in 2026.

In February 2018, we issued $500,000 of 4.75% senior unsecured notes due 2028, raising net proceeds of approximately $487,264 after underwriting discounts and expenses. We used the net proceeds of this offering to reduce amounts outstanding under our revolving credit facility.

In January 2018, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $4,338, a maturity date in September 2043 and an annual interest rate of 4.4%. In July 2018, we prepaid, at par plus accrued interest, mortgage notes secured by 12 of our properties with an aggregate outstanding principal balance of approximately $90,602, maturity dates in October 2018 and a weighted average annual interest rate of 5.0%. In September 2018, we prepaid, at par plus accrued interest, a mortgage note secured by one of our properties with an outstanding principal balance of approximately $6,325, a maturity date in January 2019 and an annual interest rate of 4.7%.

We have a $1,000,000 revolving credit facility that is available for general business purposes, including acquisitions. The maturity date of our revolving credit facility is 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. 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 requires annual interest to be paid on borrowings at the rate of LIBOR plus a premium of 120 basis points, plus a facility fee of 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. As of September 30, 2018, the annual interest rate payable on borrowings under our revolving credit facility was 3.4%. The weighted average annual interest rates for borrowings under our revolving credit facility were 3.2% and 2.5% for the three months ended September 30, 2018 and 2017, respectively, and 2.9% and 2.3% for the nine months ended September 30, 2018 and 2017, respectively. As of September 30, 2018, we had $195,000 outstanding and $805,000 available for borrowing, and as of November 5, 2018, we had $165,000 outstanding and $835,000 available for borrowing under our revolving credit facility. We incurred interest expense and other associated costs related to our revolving credit facility of $2,291 and $3,929 for the three months ended September 30, 2018 and 2017, respectively, and $7,872 and $9,888 for the nine months ended September 30, 2018 and 2017, respectively. 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.

Our $350,000 term loan matures in January 2020, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at the rate of LIBOR plus a premium of 140 basis points, subject to adjustment based upon changes to our credit ratings. At September 30, 2018, the annual interest rate payable on amounts outstanding under this term loan was 3.5%. The weighted average annual interest rate for amounts outstanding under this term loan was 3.5% and 2.6% for the three months ended September 30, 2018 and 2017, respectively, and 3.3% and 2.4% for the nine months ended September 30, 2018 and 2017, respectively. We incurred interest expense and other associated costs related to this term loan of $3,263 and $2,491 for the three months ended September 30, 2018 and 2017, respectively, and $9,106 and $6,802 for the nine months ended September 30, 2018 and 2017, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $700,000 in certain circumstances.

Our $200,000 term loan matures in September 2022, and is prepayable without penalty at any time. This term loan requires annual interest to be paid at the rate of LIBOR plus a premium of 135 basis points, subject to adjustment based upon changes to our credit ratings. At September 30, 2018, the annual interest rate payable on amounts outstanding under this term loan was 3.6%. The weighted average annual interest rate for amounts outstanding under this term loan was 3.5% and 2.7% for the three months ended September 30, 2018 and 2017, respectively, and 3.3% and 2.7% for the nine months ended September 30, 2018 and 2017, respectively. We incurred interest expense and other associated costs related to this term loan of $1,842 and $1,476 for the three months ended September 30, 2018 and 2017, respectively, and $5,144 and $4,323 for the nine months ended September 30, 2018 and 2017, respectively. This term loan includes an accordion feature under which maximum borrowings may be increased to up to $400,000 in certain circumstances.

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 September 30, 2018.