6. Debt

Unsecured Debt

At March 31, 2017, our unsecured debt consisted of $15.4 billion of senior unsecured notes of the Operating Partnership, $50.0 million outstanding under the Operating Partnership’s $4.0 billion unsecured revolving credit facility, or Credit Facility, $324.2 million outstanding under the Operating Partnership’s $3.5 billion supplemental unsecured revolving credit facility, or Supplemental Facility, and together with the Credit Facility, the Credit Facilities, and $953.6 million outstanding under the Operating Partnership’s global unsecured commercial paper note program, or Commercial Paper program. The March 31, 2017 balance on the Supplemental Facility included $199.2 million (U.S. dollar equivalent) of Yen-denominated borrowings. Foreign currency denominated borrowings under the Supplemental Facility are designated as net investment hedges of a portion of our international investments.

On March 31, 2017, we had an aggregate available borrowing capacity of $6.2 billion under the Credit Facilities. The maximum aggregate outstanding balance under the Credit Facilities during the three months ended March 31, 2017 was $437.5 million and the weighted average outstanding balance was $352.3 million. Letters of credit of $19.8 million were outstanding under the Credit Facilities as of March 31, 2017.

On March 17, 2017, the Operating Partnership amended and extended the Credit Facility. The initial borrowing capacity of $4.0 billion may be increased to $5.0 billion during its term and provides for borrowings denominated in U.S. dollars, Euros, Yen, Sterling, Canadian dollars and Australian dollars. Borrowings in currencies other than the U.S. dollar are limited to 95% of the maximum revolving credit amount, as defined. The initial maturity date of the Credit Facility was extended to June 30, 2021 and can be extended for an additional year to June 30, 2022 at our sole option, subject to our continued compliance with the terms thereof. The base interest rate on the Credit Facility was reduced to LIBOR plus 77.5 basis points from 80 basis points with a facility fee of 10 basis points.

The Supplemental Facility’s borrowing capacity of $3.50 billion may be increased to $4.25 billion during its term. The initial maturity date of the Supplemental Facility is June 30, 2019 which can be extended for an additional year to June 30, 2020 at our sole option, subject to our continued compliance with the terms thereof. The base interest rate on the Supplemental Facility is LIBOR plus 80 basis points with an additional facility fee of 10 basis points. The Supplemental Facility provides for borrowings denominated in U.S. dollars, Euro, Yen, Sterling, Canadian dollars and Australian dollars.

The Operating Partnership also has available a Commercial Paper program of $1.0 billion, or the non-U.S. dollar equivalent thereof. The Operating Partnership may issue unsecured commercial paper notes, denominated in U.S. dollars, Euros and other currencies. Notes issued in non-U.S. currencies may be issued by one or more subsidiaries of the Operating Partnership and are guaranteed by the Operating Partnership. Notes will be sold under customary terms in the U.S. and Euro commercial paper note markets and rank (either by themselves or as a result of the guarantee described above) pari passu with the Operating Partnership’s other unsecured senior indebtedness. The Commercial Paper program is supported by the Credit Facilities and, if necessary or appropriate, we may make one or more draws under either of the Credit Facilities to pay amounts outstanding from time to time on the Commercial Paper program. On March 31, 2017, we had $953.6 million outstanding under the Commercial Paper program, fully comprised of U.S. dollar denominated notes with a weighted average interest rate of 1.01%. These borrowings mature on various dates through June 26, 2017 and reduce amounts otherwise available under the Credit Facilities.

Mortgage Debt

Total mortgage indebtedness was $6.6 billion and $6.5 billion at March 31, 2017 and December 31, 2016, respectively.

Covenants

Our unsecured debt agreements contain financial and other non-financial covenants. If we were to fail to comply with these covenants, after the expiration of the applicable cure periods, the debt maturity could be accelerated or other remedies could be sought by the lender, including adjustments to the applicable interest rate. As of March 31, 2017, we were in compliance with all covenants of our unsecured debt.

At March 31, 2017, we or our subsidiaries were the borrowers under 45 non‑recourse mortgage notes secured by mortgages on 48 properties, including two separate pools of cross‑defaulted and cross‑collateralized mortgages encumbering a total of five properties. Under these cross‑default provisions, a default under any mortgage included in the cross‑defaulted pool may constitute a default under all mortgages within that pool and may lead to acceleration of the indebtedness due on each property within the pool. Certain of our secured debt instruments contain financial and other non‑financial covenants which are specific to the properties that serve as collateral for that debt. If the applicable borrower under these non-recourse mortgage notes were to fail to comply with these covenants, the lender could accelerate the debt and enforce its rights against their collateral. At March 31, 2017, the applicable borrowers under these non‑recourse mortgage notes were in compliance with all covenants where non‑compliance could individually, or giving effect to applicable cross‑default provisions in the aggregate, have a material adverse effect on our financial condition, liquidity or results of operations.

Fair Value of Debt

The carrying values of our variable‑rate mortgages and other loans approximate their fair values. We estimate the fair values of consolidated fixed‑rate mortgages using cash flows discounted at current borrowing rates and other indebtedness using cash flows discounted at current market rates. We estimate the fair values of consolidated fixed‑rate unsecured notes using quoted market prices, or, if no quoted market prices are available, we use quoted market prices for securities with similar terms and maturities. The book value of our consolidated fixed‑rate mortgages and unsecured indebtedness including commercial paper was $22.1 billion at both March 31, 2017 and December 31, 2016. The fair values of these financial instruments and the related discount rate assumptions as of March 31, 2017 and December 31, 2016 are summarized as follows:


	March 31,			December 31,
	2017			2016
Fair value of fixed-rate mortgages and unsecured indebtedness	$	22,653		$	22,703
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages		4.11	%		4.12	%
Weighted average discount rates assumed in calculation of fair value for unsecured indebtedness		3.84	%		3.83	%