Debt - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended				9 Months Ended	3 Months Ended	9 Months Ended						3 Months Ended		9 Months Ended		0 Months Ended							0 Months Ended
	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014	Oct. 31, 2013	Oct. 31, 2014 0.75% Warrants	Oct. 31, 2014 0.25% Warrants	Oct. 31, 2014 Convertible Senior Notes	Oct. 31, 2014 0.75% Convertible Senior Notes due January 15, 2015	Oct. 31, 2014 0.75% Convertible Senior Notes due January 15, 2015	Jan. 31, 2014 0.75% Convertible Senior Notes due January 15, 2015	Jan. 31, 2011 0.75% Convertible Senior Notes due January 15, 2015	Oct. 31, 2014 0.25% Convertible Senior Notes due April 1, 2018	Jan. 31, 2014 0.25% Convertible Senior Notes due April 1, 2018	Mar. 31, 2013 0.25% Convertible Senior Notes due April 1, 2018	Oct. 31, 2014 0.75% Note Hedges	Jan. 31, 2011 0.75% Note Hedges	Oct. 31, 2014 Term Loan	Jul. 11, 2013 Term Loan	Jul. 11, 2013 Term Loan Minimum Base Rate	Jul. 11, 2013 Term Loan Minimum Adjusted LIBOR	Jul. 11, 2013 Term Loan Maximum Base Rate	Jul. 11, 2013 Term Loan Maximum Adjusted LIBOR	Oct. 06, 2014 Revolving Credit Facility	Oct. 31, 2014 Revolving Credit Facility	Oct. 06, 2014 Revolving Credit Facility	Oct. 06, 2014 Revolving Credit Facility Minimum	Oct. 06, 2014 Revolving Credit Facility Minimum Base Rate	Oct. 06, 2014 Revolving Credit Facility Minimum Adjusted LIBOR	Oct. 06, 2014 Revolving Credit Facility Maximum	Oct. 06, 2014 Revolving Credit Facility Maximum Base Rate	Oct. 06, 2014 Revolving Credit Facility Maximum Adjusted LIBOR
Debt Instrument [Line Items]
Par value											$ 575,000,000			$ 1,150,000,000				$ 300,000,000
Contractual interest rate					0.75%	0.25%		0.75%	0.75%		0.75%	0.25%		0.25%	0.75%
Consecutive trading days threshold							20 days
Consecutive trading days threshold, total							30 days
Stock price trigger							130.00%
Notes price trigger							98.00%
Purchase price upon fundamental change							100.00%
Deferred tax liability											51,100,000
Estimated fair value of Company's senior notes								544,400,000	544,400,000			1,400,000,000
Fair value debt basis amount							100
Closing prices of Company's common stock	$ 63.99		$ 63.99
Excess of if-converted value over principal amount									363,000,000
Payments on convertible senior notes	89,645,000	0	387,229,000	0				89,600,000	387,200,000
Shares issued upon repayment of convertible debt								2.6	11.7						2.6
Loss on extinguishment of debt								1,300,000	10,200,000
Principal balance								181,637,000	181,637,000	568,864,000		1,150,000,000	1,150,000,000
Deferred tax asset																51,400,000
Basis spread on variable rate																			0.50%	1.50%	1.00%	2.00%					0.00%	1.00%		0.75%	1.75%
Term loan payable in quarterly installments																	7,500,000
Weighted average interest rate on Term Loan																	1.70%
Maximum borrowing capacity																									650,000,000
Proceeds from revolving credit facility, net	297,325,000	0	297,325,000	0																			300,000,000
Repayments of debt																							262,500,000
Commitment fee percentage																										0.125%			0.25%
Interest rate																								1.70%
Current borrowing capacity																								$ 300,000,000