Earnings Per Share ("EPS") (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2015	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2015
Earnings Per Share (EPS) [Abstract]
Net income	$ 78,822	$ 36,506	$ 46,394	$ 33,373	$ 96,147	$ 41,900	$ 76,412	$ 51,277	$ 195,095	$ 265,736	$ 175,172
Weighted average common shares - basic									35,194,000	35,898,000	38,172,000
Equity-based awards									234,000	199,000	182,000
Weighted average common shares - diluted									35,428,000	36,097,000	38,354,000
Basic	$ 2.24	$ 1.04	$ 1.32	$ 0.95	$ 2.74	$ 1.19	$ 2.09	$ 1.39	$ 5.54	$ 7.40	$ 4.59
Diluted	$ 2.22	$ 1.03	$ 1.31	$ 0.94	$ 2.72	$ 1.18	$ 2.09	$ 1.38	$ 5.51	$ 7.36	$ 4.57
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount									5,191	0	0