6. Debt

Unsecured Debt

At June 30, 2015, our unsecured debt consisted of $13.0 billion of senior unsecured notes of the Operating Partnership, net of discounts, $1.4 billion outstanding under the Operating Partnership's $4.0 billion unsecured revolving credit facility, or Credit Facility, $240.0 million outstanding under an unsecured term loan, and $803.0 million outstanding under the Operating Partnership's global unsecured commercial paper note program, or the Commercial Paper program. The June 30, 2015 balance on the Credit Facility included $1.0 billion (U.S. dollar equivalent) of Euro-denominated borrowings and $181.5 million (U.S. dollar equivalent) of Yen-denominated borrowings. At June 30, 2015 the outstanding amount under the Commercial Paper program was $803.0 million, of which $193.0 million was related to U.S. dollar equivalent of Euro-denominated notes. Foreign currency denominated borrowings under both the Credit Facility and Commercial Paper program are designated as net investment hedges of a portion of our international investments.

On June 30, 2015, we had an aggregate available borrowing capacity of $4.5 billion under the Credit Facility and the Operating Partnership's $2.75 billion supplemental unsecured revolving credit facility, or Supplemental Facility. The maximum aggregate outstanding balance under the two credit facilities during the six months ended June 30, 2015 was $1.8 billion and the weighted average outstanding balance was $1.2 billion. Letters of credit of $37.2 million were outstanding under the two credit facilities as of June 30, 2015.

The Credit Facility's 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 75% of the maximum revolving credit amount, as defined. The initial maturity date of the Credit Facility is June 30, 2018 and can be extended for an additional year to June 30, 2019 at our sole option. The base interest rate on the Credit Facility is LIBOR plus 80 basis points with an additional facility fee of 10 basis points.

On March 2, 2015, the Operating Partnership amended and extended the Supplemental Facility. The initial borrowing capacity of $2.0 billion has been increased to $2.75 billion, may be further increased to $3.5 billion during its term, will initially mature on June 30, 2019 and can be extended for an additional year to June 30, 2020 at our sole option. The base interest rate on the amended Supplemental Facility was reduced to LIBOR plus 80 basis points and the additional facility fee was reduced to 10 basis points. The Supplemental Facility provides for borrowings denominated in U.S. Dollars, Euro, Yen, Sterling, Canadian Dollars and Australian Dollars.

On March 2, 2015, the Operating Partnership increased the maximum aggregate program size of its Commercial Paper program from $500.0 million to $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 will rank (either by themselves or as a result of the guarantee described above) pari passu with the Operating Partnership's other unsecured senior indebtedness. Our Commercial Paper program is supported by our Credit Facility and Supplemental Facility and if necessary or appropriate, we may make one or more draws under either the Credit Facility or Supplemental Facility to pay amounts outstanding from time to time on the Commercial Paper program. At June 30, 2015, we had $803.0 million outstanding under the Commercial Paper program, comprised of $610.0 million outstanding in U.S. dollar denominated notes and $193.0 million (U.S. dollar equivalent) of Euro denominated notes with weighted average interest rates of 0.22% and 0.05%, respectively. The borrowings mature on various dates from July 1, 2015 to September 25, 2015.

During the six months ended June 30, 2015, we used cash on hand to repay at maturity $326.8 million of senior unsecured notes with a fixed rate of 5.10%.

Mortgage Debt

Total mortgage indebtedness was $6.6 billion and $6.2 billion at June 30, 2015 and December 31, 2014, respectively.

On January 15, 2015, as discussed in Note 9, we acquired two properties — Jersey Gardens in Elizabeth, New Jersey (renamed "The Mills at Jersey Gardens") and University Park Village in Fort Worth, Texas, subject to existing fixed-rate mortgage loans of $350.0 million and $55.0 million, respectively. The loans mature on November 1, 2020 and May 1, 2028 and bear interest at 3.83% and 3.85%, 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 June 30, 2015, we were in compliance with all covenants of our unsecured debt.

At June 30, 2015, we or our subsidiaries are the borrowers under 40 non-recourse mortgage notes secured by mortgages on 54 properties, including five separate pools of cross-defaulted and cross-collateralized mortgages encumbering a total of 21 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 which serve as collateral for that debt. If the borrower fails to comply with these covenants, the lender could accelerate the debt and enforce its right against their collateral. At June 30, 2015, 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, results of operations or cash flows.

Fair Value of Debt

The carrying value of our variable-rate mortgages and other loans approximates 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 was $19.0 billion as of June 30, 2015 and December 31, 2014. The fair values of these financial instruments and the related discount rate assumptions as of June 30, 2015 and December 31, 2014 are summarized as follows:

	June 30, 2015		December 31, 2014
Fair value of fixed-rate mortgages and unsecured indebtedness	$	20,135	$	20,558
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages		2.77%		3.02%