Activity for servicing assets and the related changes in fair value for the three and nine months ended September 30, 2020 and 2019 was as follows:

	Three Months Ended September 30,					Nine Months Ended September 30,
	2020		2019			2020			2019
Beginning balance	$	18,351	$	19,760		$	19,471		$	19,693
Additions, net		1,794		1,789			4,449			4,340
Changes in fair value		1,122		(1,610	)		(2,653	)		(4,094	)
Ending balance	$	21,267	$	19,939		$	21,267		$	19,939

Loans serviced for others are not included in the Consolidated Statements of Financial Condition. The unpaid principal balances of these loans serviced for others as of September 30, 2020 and December 31, 2019 were as follows:

	September 30,		December 31,
	2020		2019
Loan portfolios serviced for:
SBA guaranteed loans	$	1,333,598	$	1,231,959
USDA guaranteed loans		121,650		119,047
Total	$	1,455,248	$	1,351,006

Loan servicing revenue totaled $2.9 million and $2.7 million for the three months ended September 30, 2020 and 2019, respectively. Loan servicing revenue totaled $8.7 million and $7.9 million for the nine months ended September 30, 2020 and 2019, respectively. Loan servicing asset revaluation, which represents the changes in fair value of servicing assets, resulted in an upward valuation of $1.1 million for three months ended September 30, 2020 and a downward valuation of $1.6 million for three months ended September 30, 2019, respectively. Loan servicing asset revaluation resulted in downward valuations of $2.7 million and $4.1 million for the nine months ended September 30, 2020 and 2019, respectively.

The fair value of servicing rights is highly sensitive to changes in underlying assumptions. Changes in secondary market premiums contribute to the change in fair value of servicing rights while prepayment speed assumptions have the most significant impact on the fair value of servicing rights.

Generally, as interest rates rise on variable rate loans, loan prepayments increase due to an increase in refinance activity, which may result in a decrease in the fair value of servicing assets. Measurement of fair value is limited to the conditions existing and the assumptions used as of a particular point in time, and those assumptions may change over time. Refer to Note 15—Fair Value Measurement for further details.