8. 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 six months ended June 30, 2015:

	Core			Mortgage
	Deposit			Servicing
	Premium			Rights			Total
	(Dollars in thousands)

Balance, beginning of period	$	10,029		$	19,668		$	29,697
Additions	—			1,245			1,245
Amortization	(1,337		)	(2,327		)	(3,664		)
Balance, end of period	$	8,692		$	18,586		$	27,278

Income generated as the result of new mortgage servicing rights is reported as gains on sales of loans and totaled $0.6 million and $1.2 million for the three and six months ended June 30, 2015, respectively, compared to $0.5 million and $0.9 million for the comparable prior year periods. Amortization of mortgage servicing rights was $0.9 million and $2.3 million for the three and six months ended June 30, 2015, respectively, compared to $0.6 million and $1.2 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:

	Six Months Ended June 30,
	2015			2014
	(Dollars in thousands)

Fair market value, beginning of period	$	19,975		$	21,399
Fair market value, end of period	19,202			20,341
Weighted average discount rate	9.5		%	8.0		%
Weighted average prepayment speed assumption	13.8			15.1

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

	June 30, 2015							December 31, 2014
	Gross					Net		Gross					Net
	Carrying		Accumulated			Carrying		Carrying		Accumulated			Carrying
	Value		Amortization			Value		Value		Amortization			Value
	(Dollars in thousands)

Core deposit premium	$	44,642	$	(35,950	)	$	8,692	$	44,642	$	(34,613	)	$	10,029
Mortgage servicing rights	56,194		(37,608		)	18,586		56,687		(37,019		)	19,668
	$	100,836	$	(73,558	)	$	27,278	$	101,329	$	(71,632	)	$	29,697

Based on the core deposit premium and mortgage servicing rights held as of June 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
	Core		Mortgage
	Deposit		Servicing
	Premium		Rights		Total
	(Dollars in thousands)

2015 (remainder)	$	1,337	$	1,482	$	2,819
2016	2,674		2,322		4,996
2017	2,674		1,646		4,320
2018	2,007		1,157		3,164
2019	—		778		778
2020	—		434		434
Thereafter	—		10,767		10,767
	$	8,692	$	18,586	$	27,278

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.