Net Income Per Share (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Feb. 28, 2019	Feb. 28, 2018	Feb. 28, 2019	Feb. 28, 2018
Earnings Per Share [Abstract]
Income from continuing operations	$ 13,435	$ 41,755	$ 30,125	$ 61,011
Net income attributable to noncontrolling interests	(405)	(903)	(835)	(1,760)
Income from continuing operations attributable to SSI	13,030	40,852	29,290	59,251
Income (loss) from discontinued operations, net of tax	(138)	164	(210)	129
Net income attributable to SSI	$ 12,892	$ 41,016	$ 29,080	$ 59,380
Computation of shares:
Weighted average common shares outstanding, basic (in shares)	27,630,000	27,797,000	27,568,000	27,745,000
Incremental common shares attributable to dilutive performance share awards, DSUs, and RSUs (in shares)	484,000	1,008,000	671,000	992,000
Weighted average common shares outstanding, diluted (in shares)	28,114,000	28,805,000	28,239,000	28,737,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	313,956		159,417