
	December 31,
	2020		2019
Secured indebtedness:
4.00% mortgage loan due 2029 (1)	$	93,350	$	95,303
	93,350		95,303
Unsecured indebtedness:
3.20% (3.363% effective rate) notes due 2021 (2)	149,901		299,369
3.625% (3.752% effective rate) notes due 2023 (3)	249,464		249,201
3.875% (4.038% effective rate) notes due 2027 (4)	297,534		297,134
4.125% (4.271% effective rate) notes due 2028 (5)	347,035		346,621
4.20% (4.234% effective rate) notes due 2029 (6)	349,189		349,091
3.050% (3.079% effective rate) notes due 2030 (7)	399,106		399,009
2.600% (2.645% effective rate) notes due 2031 (8)	398,423		—
Variable rate term loan due 2022 (9)	—		100,000
Variable rate term loan due 2022 (10)	200,000		200,000
Revolving credit facility due 2022	—		221,000
	2,390,652		2,461,425
Less-unamortized debt issuance costs	(13,981)		(13,018)
Total mortgages and notes payable, net	$	2,470,021	$	2,543,710

(1)Our secured mortgage loan was collateralized by real estate assets with an undepreciated book value of $147.9 million at December 31, 2020. We paid down $2.0 million of secured loan balances through principal amortization during 2020.

(10)As more fully described in Note 7, we entered into floating-to-fixed interest rate swaps that effectively fix LIBOR for $50.0 million of this loan through January 2022. Accordingly, the equivalent fixed rate of this amount is 2.79%. The interest rate on the remaining $150.0 million was 1.25% at December 31, 2020.

Our $600.0 million unsecured revolving credit facility is scheduled to mature in January 2022 and includes an accordion feature that allows for an additional $400.0 million of borrowing capacity subject to additional lender commitments. Assuming no defaults have occurred, we have an option to extend the maturity for two additional six-month periods. The interest rate at our current credit ratings is LIBOR plus 100 basis points and the annual facility fee is 20 basis points. There were no amounts outstanding under our revolving credit facility at both December 31, 2020 and January 29, 2021. At both December 31, 2020 and January 29, 2021, we had $0.1 million of outstanding letters of credit, which reduces the availability on our revolving credit facility. As a result, the unused capacity of our revolving credit facility at both December 31, 2020 and January 29, 2021 was $599.9 million.

During 2020, the Operating Partnership issued $400.0 million aggregate principal amount of 2.600% notes due February 2031, less original issuance discount of $1.6 million. These notes were priced to yield 2.645%. Underwriting fees and other expenses were incurred that aggregated $3.4 million; these costs were deferred and will be amortized over the term of the notes. The net proceeds from the issuance were used: (1) to finance the Operating Partnership’s cash tender offer to purchase $150.0 million principal amount of its 3.20% notes due June 15, 2021 at a purchase price of 101.908% of the face amount of the notes, plus accrued and unpaid interest; (2) to prepay without penalty our $100.0 million unsecured bank term loan that was scheduled to mature in January 2022 and which bore interest at LIBOR plus 110 basis points; and (3) for general corporate purposes. We recorded $3.7 million of aggregate losses on debt extinguishment related to the repurchase of the 3.20% notes and the term loan prepayment.

During 2019, the Operating Partnership issued $400.0 million aggregate principal amount of 3.050% notes due February 2030, less original issuance discount of $1.0 million. These notes were priced to yield 3.079%. Underwriting fees and other expenses were incurred that aggregated $3.4 million; these costs were deferred and will be amortized over the term of the notes.

During 2019, the Operating Partnership issued $350.0 million aggregate principal amount of 4.20% notes due April 2029, less original issuance discount of $1.0 million. These notes were priced to yield 4.234%. Underwriting fees and other expenses were incurred that aggregated $3.1 million; these costs were deferred and will be amortized over the term of the notes.

During 2019, we prepaid without penalty the remaining $225.0 million on our seven-year unsecured bank term loan, which was scheduled to mature in June 2020. The term loan bore interest at LIBOR plus 110 basis points. We recorded $0.4 million of loss on debt extinguishment related to this prepayment.

During 2019, we prepaid without penalty $100.0 million on our $200.0 million unsecured bank term loan and recorded $0.3 million of loss on debt extinguishment related to this prepayment. During 2020, we prepaid without penalty the remaining $100.0 million upon issuance of the $400.0 million aggregate principal amount of 2.600% notes due February 2031. The term loan was scheduled to mature in January 2022 and bore interest at LIBOR plus 110 basis points.

During 2018, the Operating Partnership issued $350.0 million aggregate principal amount of 4.125% notes due March 2028, less original issuance discount of $4.1 million. These notes were priced to yield 4.271%. Underwriting fees and other expenses were incurred that aggregated $2.9 million; these costs were deferred and will be amortized over the term of the notes.

Our revolving credit facility and bank term loans require us to comply with customary operating covenants and various financial requirements. Upon an event of default on the revolving credit facility, the lenders having at least 51.0% of the total commitments under the revolving credit facility can accelerate all borrowings then outstanding, and we could be prohibited from borrowing any further amounts under our revolving credit facility, which would adversely affect our ability to fund our operations. In addition, certain of our unsecured debt agreements contain cross-default provisions giving the unsecured lenders the right to declare a default if we are in default under more than $30.0 million with respect to other loans in some circumstances.

The Operating Partnership has $149.9 million carrying amount of 2021 notes outstanding, $249.5 million carrying amount of 2023 notes outstanding, $297.5 million carrying amount of 2027 notes outstanding, $347.0 million carrying amount of 2028 notes outstanding, $349.2 million carrying amount of 2029 notes outstanding, $399.1 million carrying amount of 2030 notes outstanding and $398.4 million carrying amount of 2031 notes outstanding. The indenture that governs these outstanding notes requires us to comply with customary operating covenants and various financial ratios. The trustee or the holders of at least 25.0% in principal amount of any series of notes can accelerate the principal amount of such series upon written notice of a default that remains uncured after 60 days.

We have considered our short-term liquidity needs within one year from February 9, 2021 (the date of issuance of the annual financial statements) and the adequacy of our estimated cash flows from operating activities and other available financing sources to meet these needs. In particular, we have considered our scheduled debt maturities during such one-year period, including the remaining $150.0 million principal amount of unsecured notes that are scheduled to mature on June 15, 2021. We intend to exercise our right to redeem the remaining 3.20% notes at par on April 15, 2021. We have concluded it is probable we will meet these short-term liquidity requirements through a combination of the following:

Total interest capitalized to development and significant building and tenant improvement projects was $8.3 million, $5.6 million and $6.7 million for the years ended December 31, 2020, 2019 and 2018, respectively.