6. Goodwill and Intangible Assets

Goodwill — Goodwill by operating segment as of September 30, 2018 and changes for the nine months then ended are as follows (in thousands):

	Contract		Pressure		Directional		Oilfield
	Drilling		Pumping		Drilling		Rentals		Total
Balance at beginning of period	$	395,060		121,444	$	88,685		6,284	$	611,473
Changes to goodwill		—		—		1,000		—		1,000
Balance at end of period	$	395,060	$	121,444	$	89,685	$	6,284	$	612,473

The goodwill reflected above has increased $1.0 million from the original preliminary purchase price allocation relating to the MS Directional acquisition that was a result of a measurement period adjustment that related to a valuation adjustment to accounts payable and accrued liabilities. There were no accumulated impairment losses related to goodwill as of September 30, 2018 or December 31, 2017.

Goodwill is evaluated at least annually as of December 31, or when circumstances require, to determine if the fair value of recorded goodwill has decreased below its carrying value. For impairment testing purposes, goodwill is evaluated at the reporting unit level. The Company’s reporting units for impairment testing are its operating segments. The Company determines whether it is more likely than not that the fair value of a reporting unit is less than its carrying value after considering qualitative, market and other factors, and if this is the case, any necessary goodwill impairment is determined using a quantitative impairment test. From time to time, the Company may perform quantitative testing for goodwill impairment in lieu of performing the qualitative assessment. If the resulting fair value of goodwill is less than the carrying value of goodwill, an impairment loss would be recognized for the amount of the shortfall.

Intangible Assets — The following table presents the gross carrying amount and accumulated amortization of the intangible assets as of September 30, 2018 and December 31, 2017 (in thousands):

	September 30, 2018							December 31, 2017
	Gross					Net		Gross					Net
	Carrying		Accumulated			Carrying		Carrying		Accumulated			Carrying
	Amount		Amortization			Amount		Amount		Amortization			Amount
Customer relationships	$	26,200	$	(8,493	)		17,707	$	26,200	$	(1,943	)	$	24,257
Developed technology		55,772		(5,139	)		50,633		48,000		(1,137	)		46,863
Favorable drilling contracts		22,500		(22,336	)		164		22,500		(18,482	)		4,018
Internal use software		482		(94	)		388		482		(21	)		461
	$	104,954	$	(36,062	)	$	68,892	$	97,182	$	(21,583	)	$	75,599

Amortization expense on intangible assets of approximately $4.2 million and $6.6 million was recorded in the three months ended September 30, 2018 and 2017, respectively. Amortization expense of intangible assets of approximately $14.5 million and $16.3 million was recorded in the nine months ended September 30, 2018 and 2017, respectively.