The Company performs a qualitative, and when necessary quantitative, impairment test on its goodwill annually on August 31^st. This qualitative assessment includes reviewing factors such as macroeconomic conditions, industry and market considerations, cost factors, overall financial performance and other relevant entity-specific events. Under FASB ASC 350-10-05 Intangibles-Goodwill and Other, goodwill impairment if any, needs to be determined if the net book value of a reporting unit exceeds its estimated fair value. If goodwill of a reporting unit is determined to be impaired, the amount of impairment is measured based on the excess of the net book value of the goodwill over the implied fair value of the goodwill.

The Company performed its annual goodwill impairment valuation in each of the periods ending August 31, 2018, 2017, and 2016, and it was determined based on each year’s analysis that there was no goodwill impairment.

A summary of changes in the Company’s goodwill during the fiscal years ended September 30, 2018 and 2017 are as follows (in thousands):

Intangible assets subject to amortization consist of the following (in thousands):

	September 30,
	2018						2017
	Gross						Gross
	Carrying		Accum.				Carrying		Accum.
	Amount		Amortization		Net		Amount		Amortization		Net
Customer lists	$	358,776	$	279,990	$	78,786	$	346,784	$	264,632	$	82,152
Trade names and other intangibles		32,739		13,081		19,658		32,047		8,981		23,066
Total	$	391,515	$	293,071	$	98,444	$	378,831	$	273,613	$	105,218

Amortization expense for intangible assets was $19.6 million, $16.7 million, and $15.4 million, for the fiscal years ended September 30, 2018, 2017, and 2016, respectively. Total estimated annual amortization expense related to intangible assets subject to amortization, for the year ended September 30, 2019 and the four succeeding fiscal years ended September 30, is as follows (in thousands):

	Amount
2019	$	18,170
2020	$	16,527
2021	$	14,020
2022	$	12,003
2023	$	10,850