Inventory - Condensed Consolidated Financial Statements Adjusted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2022	Mar. 31, 2021		Mar. 31, 2020
Change in Accounting Estimate [Line Items]
Cost of sales	$ 100,957	$ 84,943	$ 92,333	$ 92,240	$ 78,430	$ 51,240	$ 56,629	$ 48,355	$ 370,473	$ 234,655	[1]	$ 209,034	[1]
Income before income taxes	27,605	12,139	24,529	27,193	11,864	2,410	21,111	15,484	91,465	50,868	[1]	57,388	[1]
Income tax expense	9,080	2,389	6,170	6,507	1,507	550	5,078	3,633	24,146	10,769	[1]	12,732	[1]
Net income	18,525	9,750	18,359	20,686	10,356	1,859	16,033	11,852	67,319	40,099	[1],[2]	45,717	[1],[3]
Less: Comprehensive income attributable to redeemable noncontrolling interest									(934)	0	[3]	0	[3]
Income attributable to redeemable noncontrolling interest	(79)	(444)	(188)	(224)	0	0	0	0	(934)	0	[1]	0	[1]
Net income attributable to CSW Industrials, Inc.	$ 18,446	$ 9,306	$ 18,171	$ 20,462	$ 10,356	$ 1,859	$ 16,033	$ 11,852	$ 66,385	$ 40,099	[1]	$ 45,717	[1]
Earnings per share attributable to CSW Industrials, Inc.
Basic (in USD per share)	$ 1.17	$ 0.59	$ 1.15	$ 1.30	$ 0.66	$ 0.12	$ 1.09	$ 0.81	$ 4.21	$ 2.67	[1]	$ 3.04	[1]
Diluted (in USD per share)	$ 1.17	$ 0.59	$ 1.15	$ 1.30	$ 0.66	$ 0.12	$ 1.08	$ 0.80	$ 4.20	$ 2.65	[1]	$ 3.01	[1]
Consolidated Statements of Comprehensive Income
Net income	$ 18,525	$ 9,750	$ 18,359	$ 20,686	$ 10,356	$ 1,859	$ 16,033	$ 11,852	$ 67,319	$ 40,099	[1],[2]	$ 45,717	[1],[3]
Net income attributable to CSW Industrials, Inc.	18,446	9,306	18,171	20,462	10,356	1,859	16,033	11,852	66,385	40,099	[1]	45,717	[1]
Total comprehensive income attributable to CSW Industrials, Inc.									67,307	45,549	[3]	45,000	[3]
As Computed Under LIFO
Change in Accounting Estimate [Line Items]
Cost of sales	101,749	86,244	92,533	92,668	79,396	50,594	56,204	48,211	373,194	234,405		208,821
Income before income taxes	26,813	10,837	24,329	26,765	10,898	3,056	21,536	15,628	88,744	51,118		57,601
Income tax expense	8,835	2,068	6,121	6,401	1,270	709	5,182	3,668	23,426	10,830		12,784
Net income	17,979	8,769	18,208	20,363	9,628	2,346	16,353	11,960	65,318	40,287		45,877
Less: Comprehensive income attributable to redeemable noncontrolling interest									(1,073)
Income attributable to redeemable noncontrolling interest	(88)	(458)	(212)	(315)	0	0	0	0	(1,073)
Net income attributable to CSW Industrials, Inc.	$ 17,891	$ 8,311	$ 17,995	$ 20,048	$ 9,628	$ 2,346	$ 16,353	$ 11,960	$ 64,245	$ 40,287		$ 45,877
Earnings per share attributable to CSW Industrials, Inc.
Basic (in USD per share)	$ 1.13	$ 0.53	$ 1.14	$ 1.28	$ 0.62	$ 0.16	$ 1.11	$ 0.81	$ 4.08	$ 2.68		$ 3.05
Diluted (in USD per share)	$ 1.13	$ 0.52	$ 1.14	$ 1.27	$ 0.61	$ 0.16	$ 1.10	$ 0.81	$ 4.06	$ 2.66		$ 3.02
Consolidated Statements of Comprehensive Income
Net income	$ 17,979	$ 8,769	$ 18,208	$ 20,363	$ 9,628	$ 2,346	$ 16,353	$ 11,960	$ 65,318	$ 40,287		$ 45,877
Net income attributable to CSW Industrials, Inc.	17,891	8,311	17,995	20,048	9,628	2,346	16,353	11,960	64,245	40,287		45,877
Total comprehensive income attributable to CSW Industrials, Inc.									65,167	45,738		45,160
Effect of Change
Change in Accounting Estimate [Line Items]
Cost of sales	(792)	(1,301)	(200)	(428)	(966)	646	425	144	(2,721)	250		213
Income before income taxes	792	1,302	200	428	966	(646)	(425)	(144)	2,721	(250)		(213)
Income tax expense	245	321	49	106	237	(159)	(104)	(35)	720	(61)		(52)
Net income	546	981	151	323	728	(487)	(320)	(108)	2,001	(188)		(160)
Less: Comprehensive income attributable to redeemable noncontrolling interest									139
Income attributable to redeemable noncontrolling interest	9	14	24	91	0	0	0	0	139
Net income attributable to CSW Industrials, Inc.	$ 555	$ 995	$ 176	$ 414	$ 728	$ (487)	$ (320)	$ (108)	$ 2,140	$ (188)		$ (160)
Earnings per share attributable to CSW Industrials, Inc.
Basic (in USD per share)	$ 0.04	$ 0.06	$ 0.01	$ 0.02	$ 0.04	$ (0.04)	$ (0.02)	$ 0	$ 0.13	$ (0.01)		$ (0.01)
Diluted (in USD per share)	$ 0.04	$ 0.07	$ 0.01	$ 0.03	$ 0.05	$ (0.04)	$ (0.02)	$ (0.01)	$ 0.14	$ (0.01)		$ (0.01)
Consolidated Statements of Comprehensive Income
Net income	$ 546	$ 981	$ 151	$ 323	$ 728	$ (487)	$ (320)	$ (108)	$ 2,001	$ (188)		$ (160)
Net income attributable to CSW Industrials, Inc.	$ 555	$ 995	$ 176	$ 414	$ 728	$ (487)	$ (320)	$ (108)	2,140	(188)		(160)
Total comprehensive income attributable to CSW Industrials, Inc.									$ 2,140	$ (189)		$ (160)

[1] *Years ended March 31, 2021 and 2020 amounts have been adjusted to reflect the change in inventory accounting method, as described in Notes 1 and 7 to the Consolidated Financial Statements.

[2] Years ended March 31, 2021 and 2020 amounts have been adjusted to reflect the change in inventory accounting method, as described in Notes 1 and 7 to the Consolidated Financial Statements.

[3] Years ended March 31, 2021 and 2020 amounts have been adjusted to reflect the change in inventory accounting method, as described in Notes 1 and 7 to the Consolidated Financial Statements.