OTHER INTANGIBLE ASSETS

Other intangible assets include a core deposit premium and mortgage servicing rights. The following table presents changes in other intangible assets for the nine months ended September 30, 2015:



(dollars in thousands)	Core Deposit Premium			Mortgage Servicing Rights			Total
Balance, beginning of period	$	10,029		$	19,668		$	29,697
Additions	—			1,847			1,847
Amortization	(2,006		)	(3,341		)	(5,347		)
Balance, end of period	$	8,023		$	18,174		$	26,197

Income generated as the result of new mortgage servicing rights is reported as gains on sales of loans and totaled $0.6 million and $1.8 million for the three and nine months ended September 30, 2015, respectively, compared to $0.7 million and $1.6 million for the comparable prior year periods. Amortization of mortgage servicing rights was $1.0 million and $3.3 million for the three and nine months ended September 30, 2015, respectively, compared to $0.7 million and $1.9 million for the comparable prior year periods.

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 September 30,
(dollars in thousands)	2015			2014
Fair market value, beginning of period	$	19,975		$	21,399
Fair market value, end of period	18,495			20,471
Weighted average discount rate	9.5		%	9.5		%
Weighted average prepayment speed assumption	14.3			12.9

The gross carrying value and accumulated amortization related to our intangible assets are presented below:



	September 30, 2015							December 31, 2014
(dollars in thousands)	Gross Carrying Value		Accumulated Amortization			Net Carrying Value		Gross Carrying Value		Accumulated Amortization			Net Carrying Value
Core deposit premium	$	44,642	$	(36,619	)	$	8,023	$	44,642	$	(34,613	)	$	10,029
Mortgage servicing rights	58,534		(40,360		)	18,174		56,687		(37,019		)	19,668
	$	103,176	$	(76,979	)	$	26,197	$	101,329	$	(71,632	)	$	29,697

Based on the core deposit premium and mortgage servicing rights held as of September 30, 2015, estimated amortization expense for the remainder of fiscal year 2015, the next five succeeding fiscal years and all years thereafter are as follows:



	Estimated Amortization Expense
(dollars in thousands)	Core Deposit Premium		Mortgage Servicing Rights		Total
2015 (remainder)	$	669	$	1,159	$	1,828
2016	2,674		4,164		6,838
2017	2,674		3,156		5,830
2018	2,006		2,488		4,494
2019	—		1,906		1,906
2020	—		1,492		1,492
Thereafter	—		3,809		3,809
	$	8,023	$	18,174	$	26,197

We perform an impairment assessment of our other intangible assets whenever events or changes in circumstance indicate that the carrying value of those assets may not be recoverable. Our impairment assessments involve, among other valuation methods, the estimation of future cash flows and other methods of determining fair value. Estimating future cash flows and determining fair values is subject to judgment and often involves the use of significant estimates and assumptions. The variability of the factors we use to perform our impairment tests depend on a number of conditions, including the uncertainty about future events and cash flows. All such factors are interdependent and, therefore, do not change in isolation. Accordingly, our accounting estimates may materially change from period to period due to changing market factors.