COMPUTATION OF EARNINGS PER COMMON SHARE (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Earnings Per Share [Abstract]
Net income	$ 25,824	$ 30,384	$ 27,421	$ 28,672	$ 27,943	$ 34,383	$ 23,138	$ 26,953	$ 112,301	$ 112,417	$ 106,402
Earnings Per Share, Basic and Diluted
Weighted average common shares-basic									41,723	42,452	41,748
Dilutive effect of employee equity incentive plans									920	912	990
Weighted average common shares-diluted									42,643	43,364	42,738
Basic net income per common share (in dollars per share)	$ 0.62	$ 0.73	$ 0.66	$ 0.69	$ 0.67	$ 0.81	$ 0.54	$ 0.63	$ 2.69	$ 2.65	$ 2.55
Diluted net income per common share (in dollars per share)	$ 0.61	$ 0.71	$ 0.65	$ 0.68	$ 0.65	$ 0.80	$ 0.53	$ 0.62	$ 2.63	$ 2.59	$ 2.49
Potentially dilutive securities excluded from diluted earnings per share because their effect is anti-dilutive									442	202	1,038