6. Mortgage Notes Payable

The Company had outstanding mortgage notes payable totaling approximately $3.4 billion and $4.3 billion as of December 31, 2015 and 2014, respectively, each collateralized by one or more buildings and related land included in real estate assets. The mortgage notes payable are generally due in monthly installments and mature at various dates through April 10, 2022.

Fixed rate mortgage notes payable totaled approximately $3.4 billion and $4.3 billion at December 31, 2015 and 2014, respectively, with contractual interest rates ranging from 4.75% to 7.69% per annum at December 31, 2015 and 2014 (with a weighted-average of 5.69% and 5.70% (excluding the mezzanine notes payable) at December 31, 2015 and 2014, respectively).

There were no variable rate mortgage loans at December 31, 2015 and 2014. As of December 31, 2015 and 2014, the LIBOR rate was 0.43% and 0.17%, respectively.

On September 18, 2015, in connection with the sale of 505 9th Street, N.W. located in Washington, DC by a consolidated entity in which the Company has a 50% interest, the consolidated entity assigned to the buyer the mortgage loan collateralized by the property totaling approximately $117.0 million. The assigned mortgage loan bears interest at a fixed rate of 5.73% per annum and matures on November 1, 2017 (See Note 3).

On October 1, 2015, the Company used available cash to repay the mortgage loan collateralized by its Kingstowne Two and Kingstowne Retail properties located in Alexandria, Virginia totaling approximately $29.8 million. The mortgage loan bore interest at a fixed rate of 5.99% per annum and was scheduled to mature on January 1, 2016. There was no prepayment penalty.

On December 15, 2015, the Company legally defeased the mortgage loan collateralized by its 100 & 200 Clarendon Street (formerly known as the John Hancock Tower and Garage) properties located in Boston, Massachusetts. The mortgage loan had an outstanding principal balance of $640.5 million, bore interest at a fixed rate of 5.68% per annum and was scheduled to mature on January 6, 2017. The cash outlay required for the defeasance in the net amount of approximately $667.3 million was based on the purchase price of U.S. government securities that will generate sufficient cash flow to fund continued interest payments on the loan from the effective date of the defeasance through, and the repayment of the loan on, October 6, 2016, which is the date on which the Company could repay the loan at par. In connection with the defeasance, the mortgage and other liens on the property were extinguished and all existing collateral, including various guarantees, were released. As a result of the defeasance, the Company recognized a loss from early extinguishment of debt of approximately $22.0 million, consisting of approximately $26.8 million, which is the difference between the purchase price for the U.S. government securities acquired for the defeasance and the outstanding principal balance of the mortgage loan, and approximately $1.4 million of unamortized deferred financing costs, offset by approximately $4.8 million from the acceleration of the remaining balance of the historical fair value debt adjustment and approximately $1.4 million of accrued interest expense through the effective date of the defeasance.

Two mortgage loans totaling approximately $1.5 billion at December 31, 2015 and four mortgage loans totaling approximately $2.2 billion at December 31, 2014 have been accounted for at their fair values on the dates the mortgage loans were assumed in connection with acquisitions of real estate. The impact of recording the mortgage loans at fair value resulted in a decrease to interest expense of approximately $55.0 million, $52.5 million and $34.4 million for the years ended December 31, 2015, 2014 and 2013, respectively. The cumulative liability related to the fair value adjustments was $80.2 million and $138.7 million at December 31, 2015 and 2014, respectively, and is included in mortgage notes payable in the Consolidated Balance Sheets.

Contractual aggregate principal payments of mortgage notes payable at December 31, 2015 are as follows:



	Principal Payments
	(in thousands)
2016	$	576,864
2017	2,067,654
2018	18,633
2019	19,670
2020	20,766
Thereafter	654,892
Total aggregate principal payments	3,358,479
Unamortized balance of historical fair value adjustments	80,235
Total carrying value of mortgage notes payable	$	3,438,714