Earnings Per Share ("EPS") (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2015	Oct. 31, 2014	Jul. 31, 2014	Apr. 30, 2014	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2014
Earnings per share, basic and diluted [Line Items]
Net income									$ 265,736	$ 175,172	$ 179,932
Weighted average common shares - basic									35,898,000	38,172,000	38,020,000
Equity-based awards									199,000	182,000	208,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									0	0	6,236
Weighted average common shares - diluted									36,097,000	38,354,000	38,228,000
Earnings Per Share, Basic	$ 2.74	$ 1.19	$ 2.09	$ 1.39	$ 2.12	$ 1.09	$ 1.03	$ 0.35	$ 7.40	$ 4.59	$ 4.73
Earnings Per Share, Diluted	$ 2.72	$ 1.18	$ 2.09	$ 1.38	$ 2.11	$ 1.09	$ 1.03	$ 0.35	$ 7.36	$ 4.57	$ 4.71