Earnings Per Share ("EPS") (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended										12 Months Ended
	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019		Oct. 31, 2018		Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Net income	$ 149,080	$ 90,770	$ 79,250	$ 55,400	$ 116,799	[1]	$ 114,216	[1]	$ 75,866	$ 33,699	$ 374,500	$ 340,580	$ 116,641
Weighted average common shares - basic											36,186,000	38,094,000	37,957,000
Equity-based awards											274,000	223,000	259,000
Weighted average common shares - diluted											36,460,000	38,317,000	38,216,000
Basic	$ 4.19	$ 2.54	$ 2.17	$ 1.50	$ 3.13		$ 2.98		$ 1.97	$ 0.88	$ 10.35	$ 8.94	$ 3.07
Diluted	$ 4.15	$ 2.52	$ 2.16	$ 1.49	$ 3.11		$ 2.96		$ 1.97	$ 0.87	$ 10.27	$ 8.89	$ 3.05
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount											0	21,008	3,017
Retained earnings
Net income											$ 374,500	$ 340,580	$ 116,641

[1] The Company decreased its estimate of the one-time transition tax related to the enactment of U.S. Tax Reform by $24.0 million and $25.2 million, respectively, in the third and fourth quarters of fiscal 2019 (see Note 9 – Income Taxes for further discussion).