6. Debt

Unsecured Debt

At March 31, 2014, our unsecured debt consisted of $14.4 billion of senior unsecured notes of the Operating Partnership, net of discounts, $657.4 million outstanding under the Operating Partnership's $4.0 billion unsecured revolving credit facility, or Credit Facility, $216.4 million outstanding under the Operating Partnership's $2.0 billion supplemental unsecured revolving credit facility, or Supplemental Facility, and $240.0 million outstanding under an unsecured term loan. At March 31, 2014, the Credit Facility had a capacity of $4.0 billion including a $2.0 billion multi-currency tranche, an initial maturity of October 30, 2015, an interest rate of LIBOR plus 95 basis points and an additional facility fee of 15 basis points. In addition, the Credit Facility provides for a money-market competitive bid option program that allows us to hold auctions to achieve lower pricing for short term borrowings. The entire balance on the Credit Facility at March 31, 2014 consisted of Euro-denominated borrowings and the entire balance on the Supplemental Facility on such date consisted of Yen-denominated borrowings, both of which are designated as net investment hedges of our international investments.

On March 31, 2014, we had an aggregate available borrowing capacity of $5.1 billion under the two credit facilities. The maximum outstanding balance of the credit facilities during the three months ended March 31, 2014 was $1.2 billion and the weighted average outstanding balance was $973.2 million. Letters of credit of $41.7 million were outstanding under the two credit facilities as of March 31, 2014.

On April 7, 2014, the Operating Partnership amended and extended the Credit Facility. The initial borrowing capacity of $4.0 billion can now 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 was extended to June 30, 2018 and can be extended for an additional year at our sole option. The base interest rate on the amended Credit Facility was reduced to LIBOR plus 80 basis points with the additional facility fee reduced to 10 basis points.

The Supplemental Facility's borrowing capacity of $2.0 billion can be increased at our sole option to $2.5 billion during its term. The Supplemental Facility will initially mature on June 30, 2016 and can be extended for an additional year at our sole option. As of March 31, 2014, the base interest rate on the Supplemental Facility was LIBOR plus 95 basis points with an additional facility fee of 15 basis points. Like the Credit Facility, the Supplemental Facility provides for a money market competitive bid option program and allows for multi-currency borrowings.

On January 21, 2014, the Operating Partnership issued $600.0 million of senior unsecured notes at a fixed interest rate of 2.20% with a maturity date of February 1, 2019 and $600.0 million of senior unsecured notes at a fixed interest rate of 3.75% with a maturity date of February 1, 2024. Proceeds from the unsecured notes offering were used to repay debt and for general corporate purposes.

During the three months ended March 31, 2014, we used cash on hand to redeem at par or repay at maturity $716.1 million of senior unsecured notes with fixed rates ranging from 4.9% to 6.75%.

Mortgage Debt

Total mortgage indebtedness was $7.6 billion and $8.2 billion at March 31, 2014 and December 31, 2013, respectively. During the three months ended March 31, 2014, we added $370.0 million in new mortgage loans on six previously unencumbered properties which are expected to be part of the Washington Prime portfolio with a weighted average interest rate of 4.60%.

On January 2, 2014, we repaid the $820.0 million outstanding mortgage at Sawgrass Mills originally maturing July 1, 2014 and on February 28 2014, we repaid the $269.0 million outstanding mortgage at Great Mall originally maturing August 28, 2015.

Covenants

Our unsecured debt agreements contain financial and other 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, 2014, we were in compliance with all covenants of our unsecured debt.

At March 31, 2014, we or our subsidiaries are the borrowers under 66 non-recourse mortgage notes secured by mortgages on 85 properties, including eight separate pools of cross-defaulted and cross-collateralized mortgages encumbering a total of 29 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 March 31, 2014, 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 $21.7 billion and $21.8 billion as of March 31, 2014 and December 31, 2013, respectively. The fair values of these financial instruments and the related discount rate assumptions as of March 31, 2014 and December 31, 2013 are summarized as follows:

	March 31, 2014	December 31, 2013

Fair value of fixed-rate mortgages and unsecured indebtedness	$23,433	$23,297
Weighted average discount rates assumed in calculation of fair value for fixed-rate mortgages	3.35%	3.07%