7. CORE DEPOSIT PREMIUM AND MORTGAGE SERVICING RIGHTS

The following table presents changes in core deposit premium and mortgage servicing rights for the periods presented:



(dollars in thousands)	Core Deposit Premium			Mortgage Servicing Rights			Total
Balance, January 1, 2017	$	4,680		$	15,779		$	20,459
Additions	—			1,857			1,857
Amortization	(2,006		)	(1,543		)	(3,549		)
Balance, September 30, 2017	$	2,674		$	16,093		$	18,767

Balance, January 1, 2018	$	2,006		$	15,843		$	17,849
Additions	—			1,204			1,204
Amortization	(2,006		)	(1,413		)	(3,419		)
Balance, September 30, 2018	$	—		$	15,634		$	15,634

Income generated as the result of new mortgage servicing rights is reported as gains on sales of loans and totaled $0.4 million and $1.2 million for the three and nine months ended September 30, 2018, respectively, compared to $0.6 million and $1.9 million for the three and nine months ended September 30, 2017, respectively.

Amortization of mortgage servicing rights was $0.5 million and $1.4 million, respectively, for the three and nine months ended September 30, 2018, compared to $0.5 million and $1.5 million for the three and nine months ended September 30, 2017, respectively.

The following table presents the fair market value and key assumptions used in determining the fair market value of our mortgage servicing rights:



	Nine Months Ended			Nine Months Ended
(dollars in thousands)	September 30, 2018			September 30, 2017
Fair market value, beginning of period	$	17,161		$	18,087
Fair market value, end of period	18,315			16,777
Weighted average discount rate	9.5		%	9.5		%
Forecasted constant prepayment rate assumption	14.0			16.5

The gross carrying value and accumulated amortization related to our core deposit premium and mortgage servicing rights are presented below:



	September 30, 2018							December 31, 2017
(dollars in thousands)	Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Core deposit premium	$	44,642	$	(44,642	)	$	—	$	44,642	$	(42,636	)	$	2,006
Mortgage servicing rights	65,605		(49,971		)	15,634		64,401		(48,558		)	15,843
Total	$	110,247	$	(94,613	)	$	15,634	$	109,043	$	(91,194	)	$	17,849

Based on the mortgage servicing rights held as of September 30, 2018, estimated amortization expense for the remainder of fiscal year 2018, the next five succeeding fiscal years and all years thereafter are as follows:



(dollars in thousands)
2018 (remainder)	$	450
2019	1,566
2020	1,292
2021	1,087
2022	919
2023	789
Thereafter	9,531
	$	15,634

We perform an impairment assessment of our mortgage servicing rights whenever events or changes in circumstance indicate that the carrying value of the asset may not be recoverable.