At December 31, 2019, loan servicing rights derived from loans sold with servicing retained totaled $25.4 million, compared to $24.5 million at December 31, 2018. Loans serviced for others are not reported as assets. The principal balance of loans serviced for others was $3.445 billion at December 31, 2019, compared to $3.306 billion at December 31, 2018. Approximately 99.8% of the loans serviced for others at December 31, 2019 were residential mortgage loans. Custodial escrow balances maintained in connection with serviced loans were $12.7 million at December 31, 2019 and $10.7 million at December 31, 2018.

The following table summarizes the activity related to loan servicing rights and the related valuation allowance in 2019, 2018, and 2017:

	Years Ended December 31,
(dollars in thousands)	2019			2018			2017
Balance at beginning of period	$	24,512		$	24,690		$	25,629
Additions (1)		6,499			4,264			4,206
Amortization		(5,612	)		(4,442	)		(5,145	)
Balance before valuation allowance at end of period		25,399			24,512			24,690
Valuation allowance:
Balance at beginning of period		(15	)		(29	)		(68	)
(Additions)/recoveries		(16	)		14			39
Balance at end of period		(31	)		(15	)		(29	)
Loan servicing rights, net	$	25,368		$	24,497		$	24,661

At December 31, 2019, the fair value of servicing rights was $26.5 million, which was determined using a discount rate of 12% and a weighted average prepayment speed of 167% PSA. At December 31, 2018, the fair value of servicing rights was $27.4 million, which was determined using a discount rate of 12% and a weighted average prepayment speed of 119% PSA.