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Secured and unsecured senior debt (Notes)
 12 Months Ended
Dec. 31, 2023
Debt Disclosure [Abstract]  
Secured and unsecured senior debt
The following table summarizes our outstanding indebtedness and respective principal payments as of December 31, 2023 (dollars in thousands):
Stated 
Rate
Interest Rate (1)
Maturity Date (2)
Principal Payments Remaining for the Periods Ending December 31,Unamortized (Deferred Financing Cost), (Discount) Premium
Debt20242025202620272028ThereafterPrincipalTotal
Secured notes payable
Greater Boston(3)
SOFR+2.70 %8.38 %11/19/26$— $— $119,674 $— $— $— $119,674 $(631)$119,043 
San Francisco Bay Area6.50 %6.50 7/1/3632 34 36 38 41 438 619 — 619 
Secured debt weighted-average interest rate/subtotal8.37 32 34 119,710 38 41 438 120,293 (631)119,662 
Unsecured senior line of credit and commercial paper program(4)
(4)
5.76 
(4)
1/22/28
(4)
(4)
— — — 100,000 — 
(4)
100,000 (48)99,952 
Unsecured senior notes payable3.45 %3.62 4/30/25— 600,000 — — — — 600,000 (1,181)598,819 
Unsecured senior notes payable4.30 %4.50 1/15/26— — 300,000 — — — 300,000 (1,022)298,978 
Unsecured senior notes payable3.80 %3.96 4/15/26— — 350,000 — — — 350,000 (1,143)348,857 
Unsecured senior notes payable3.95 %4.13 1/15/27— — — 350,000 — — 350,000 (1,574)348,426 
Unsecured senior notes payable3.95 %4.07 1/15/28— — — — 425,000 — 425,000 (1,733)423,267 
Unsecured senior notes payable4.50 %4.60 7/30/29— — — — — 300,000 300,000 (1,248)298,752 
Unsecured senior notes payable2.75 %2.87 12/15/29— — — — — 400,000 400,000 (2,473)397,527 
Unsecured senior notes payable4.70 %4.81 7/1/30— — — — — 450,000 450,000 (2,425)447,575 
Unsecured senior notes payable4.90 %5.05 12/15/30— — — — — 700,000 700,000 (5,511)694,489 
Unsecured senior notes payable3.375 %3.48 8/15/31— — — — — 750,000 750,000 (4,990)745,010 
Unsecured senior notes payable2.00 %2.12 5/18/32— — — — — 900,000 900,000 (7,887)892,113 
Unsecured senior notes payable1.875 %1.97 2/1/33— — — — — 1,000,000 1,000,000 (7,976)992,024 
Unsecured senior notes payable2.95 %3.07 3/15/34— — — — — 800,000 800,000 (7,989)792,011 
Unsecured senior notes payable4.75 %4.88 4/15/35— — — — — 500,000 500,000 (5,411)494,589 
Unsecured senior notes payable4.85 %4.93 4/15/49— — — — — 300,000 300,000 (2,987)297,013 
Unsecured senior notes payable4.00 %3.91 2/1/50— — — — — 700,000 700,000 10,111 710,111 
Unsecured senior notes payable3.00 %3.08 5/18/51— — — — — 850,000 850,000 (11,608)838,392 
Unsecured senior notes payable3.55 %3.63 3/15/52— — — — — 1,000,000 1,000,000 (14,112)985,888 
Unsecured senior notes payable5.15 %5.26 4/15/53— — — — — 500,000 500,000 (7,813)492,187 
Unsecured debt weighted-average interest rate/subtotal3.67 — 600,000 650,000 350,000 525,000 9,150,000 11,275,000 (79,020)11,195,980 
Weighted-average interest rate/total 3.72 %$32 $600,034 $769,710 $350,038 $525,041 $9,150,438 $11,395,293 $(79,651)$11,315,642 

(1)Represents the weighted-average interest rate as of the end of the applicable period, including amortization of loan fees, amortization of debt premiums (discounts), and other bank fees.
(2)Reflects any extension options that we control.
(3)Represents a secured construction loan held by our consolidated real estate joint venture at 99 Coolidge Avenue, of which we own a 75.0% interest. As of December 31, 2023, this joint venture has $75.6 million available under existing lender commitments. The interest rate shall be reduced from SOFR+2.70% to SOFR+2.10% over time upon the completion of certain leasing, construction, and financial covenant milestones.
(4)Refer to “$5.0 billion unsecured senior line of credit” and “$2.5 billion commercial paper program” on the following page.
The following table summarizes our secured and unsecured senior debt and amounts outstanding under our unsecured senior line of credit and commercial paper program as of December 31, 2023 (dollars in thousands):
Fixed-Rate Debt
Variable-Rate Debt		Weighted-Average
Interest Rate(1)
Remaining Term
(in years)
TotalPercentage
Secured notes payable$619 $119,043 $119,662 1.1 %8.37 %2.9
Unsecured senior notes payable11,096,028 — 11,096,028 98.0 3.65 13.0
Unsecured senior line of credit and commercial paper program— 99,952 99,952 
(2)
0.9 5.76 
(2)
4.1
(3)
Total/weighted average$11,096,647 $218,995 $11,315,642 100.0 %3.72 %12.8
(3)
Percentage of total debt98.1 %1.9 %100 %
(1)Represents the weighted-average interest rate as of the end of the applicable period, including expense/income related to the amortization of loan fees, amortization of debt premiums (discounts), and other bank fees.
(2)As of December 31, 2023, we had no outstanding balance on our unsecured senior line of credit and $100.0 million of commercial paper notes outstanding.
(3)We calculate the weighted-average remaining term of our commercial paper notes by using the maturity date of our unsecured senior line of credit. Using the maturity date of our outstanding commercial paper notes, the consolidated weighted-average maturity of our debt is 12.7 years. The commercial paper notes sold during the year ended December 31, 2023 were issued at a weighted-average yield to maturity of 5.55% and had a weighted-average maturity term of 11 days.
Unsecured senior notes payable

In February 2023, we opportunistically issued $1.0 billion of unsecured senior notes payable with a weighted-average interest rate of 4.95% and a weighted-average maturity of 21.2 years. The unsecured senior notes consisted of $500.0 million of 4.75% unsecured senior notes due 2035 and $500.0 million of 5.15% unsecured senior notes due 2053.
$5.0 billion unsecured senior line of credit

In June 2023, we amended our unsecured senior line of credit to increase the aggregate commitments available for borrowing to $5.0 billion from $4.0 billion. As of December 31, 2023, we had no outstanding balance on our unsecured line of credit.

Based upon our ability to achieve certain sustainability targets, as described in our unsecured senior line of credit agreement, the interest rate and facility fee rate are subject to adjustments of up to four and one basis points, respectively. Upon meeting certain annual sustainability targets, our borrowing rate for a one-year period was reduced by four basis points to SOFR plus 0.835%, from SOFR plus 0.875%, and the facility fee was reduced by one basis point to 0.14% from 0.15% during the year ended December 31, 2023.
$2.5 billion commercial paper program

In July 2023, we increased the aggregate amount we may issue from time to time under our commercial paper program to $2.5 billion from $2.0 billion.

Our commercial paper program provides us with the ability to issue up to $2.5 billion of commercial paper notes that bear interest at short-term fixed rates with a maturity of generally 30 days or less and a maximum maturity of 397 days from the date of issuance. Our commercial paper program is backed by our unsecured senior line of credit, and at all times we expect to retain a minimum undrawn amount of borrowing capacity under our unsecured senior line of credit equal to any outstanding notes issued under our commercial paper program. We use the net proceeds from the issuances of the notes for general working capital and other general corporate purposes. General corporate purposes may include, but are not limited to, the repayment of other debt and selective development, redevelopment, or acquisition of properties. The commercial paper notes sold during the year ended December 31, 2023 were issued at a weighted-average yield to maturity of 5.55% and had a weighted-average maturity term of 11 days. As of December 31, 2023, we had $100.0 million of commercial paper notes outstanding.
Interest expense

The following table summarizes interest expense for the years ended December 31, 2023, 2022, and 2021 (in thousands):
Year Ended December 31,
202320222021
Interest incurred$438,182 $372,848 $312,806 
Capitalized interest(363,978)(278,645)(170,641)
Interest expense$74,204 $94,203 $142,165