Loans serviced for others are not included in the accompanying consolidated balance sheet. The unpaid principal balances of loans serviced for others requiring recognition of a servicing asset were $2.26 billion, $2.63 billion and $2.44 billion at December 31, 2019, 2018 and 2017, respectively. The unpaid principal balance for all loans serviced for others was $2.97 billion, $3.22 billion and $2.85 billion at December 31, 2019, 2018 and 2017, respectively.

	2019			2018
Balance at beginning of period	$	47,641		$	52,298
Additions, net		4,305			16,568
Fair value changes:
Due to changes in valuation inputs or assumptions		(3,127	)		(7,238	)
Decay due to increases in principal paydowns or runoff		(13,454	)		(13,987	)
Balance at end of period	$	35,365		$	47,641

The fair value of servicing rights was determined using a weighted average discount rate of 14.1% on December 31, 2019 and 14.5% on December 31, 2018. The fair value of servicing rights was determined using a weighted average prepayment speed of 16.4% on December 31, 2019 and 12.0% on December 31, 2018. Changes to fair value are reported in loan servicing asset revaluation within the consolidated statements of income.

The fair value of servicing rights is highly sensitive to changes in underlying assumptions. Changes in 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 results 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 not be appropriate if they are applied at a different time.