Stock-Based Compensation - Additional Information (Detail) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended									12 Months Ended									12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 2003 Equity Incentive Plan [Member]	Dec. 31, 2011 2003 Equity Incentive Plan [Member]	Dec. 31, 2012 2003 Equity incentive plan amendment.[Member]	Dec. 31, 2010 2003 Equity incentive plan amendment.[Member]	May 09, 2012 2003 Equity incentive plan amendment.[Member]	May 12, 2010 2003 Equity incentive plan amendment.[Member]	Dec. 31, 2012 Employee Stock Purchase Plans [Member]	Dec. 31, 2011 Employee Stock Purchase Plans [Member]	Dec. 31, 2010 Employee Stock Purchase Plans [Member]	Dec. 31, 2012 Service-based Restricted Stock Units [Member]	Dec. 31, 2011 Service-based Restricted Stock Units [Member]	Dec. 31, 2010 Service-based Restricted Stock Units [Member]	May 09, 2012 Restricted stock and/or restricted stock units [Member] 2003 Equity incentive plan amendment.[Member]	May 12, 2010 Restricted stock and/or restricted stock units [Member] 2003 Equity incentive plan amendment.[Member]	May 12, 2010 Two Thousand Three Equity Incentive Plan Amendment, Employee Stock Purchase Plan [Member]	Dec. 31, 2012 Employee Stock Purchase Plan [Member]	Dec. 31, 2012 Stock options and RSU's assumed from acquisition [Member]	Dec. 31, 2012 Additional vesting [Member]	Dec. 31, 2012 Employee Stock Option [Member]	Dec. 31, 2011 Employee Stock Option [Member]	Dec. 31, 2010 Employee Stock Option [Member]	Dec. 31, 2012 Stock Option Assumed In Acquisition [Member]	Dec. 31, 2012 Options Assumed In SandForce Acquisition [Member]	Dec. 31, 2012 Performance Based Restricted Stock Units [Member]	Dec. 31, 2011 Performance Based Restricted Stock Units [Member]	Dec. 31, 2010 Performance Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Description of share based payment award					Under the 2003 Plan, the Company may grant stock options and stock appreciation rights with an exercise price that is no less than the fair market value of the stock on the date of grant. Under the 2003 Plan, the Company may also grant restricted stock and restricted stock unit awards. The Company typically grants restricted stock units (“RSUs”). No participant may be granted stock options covering more than four million shares of stock or more than an aggregate of one million shares of restricted stock and RSUs in any fiscal year. The term of each option or RSU is determined by the Board of Directors or its delegate and, for option grants on or after February 12, 2004, is generally seven years. Options generally vest in equal annual installments over a four-year period.	On May 9, 2012, the 2003 Plan was amended to increase the number of shares available for new awards to 25 million shares, of which 15 million shares were available for restricted stock and/or RSUs. In addition, the period during which incentive stock options can be granted was extended to February 9, 2022, and the maximum number of shares that may be issued upon exercise of incentive stock options was set at 25 million.	On May 12, 2010, the 2003 Plan was amended to increase the shares available for new awards to 45 million shares, with 30 million shares for restricted stock and RSUs, and to update the performance measures that can be used to determine the vesting of awards intended to qualify for deductibility under Section 162(m) of the Internal Revenue Code to better match the metrics the Company uses to manage its business.			Under the ESPP, rights are granted to LSI employees to purchase shares of common stock at 85% of the lesser of the fair market value of such shares at the beginning of a 12-month offering period or the end of each six-month purchase period within such an offering period. On May 12, 2010, the ESPP was amended to increase the shares available for issuance under the plan to a total of 30 million shares and to extend the term of the ESPP through May 12, 2020. As of December 31, 2012, the ESPP had approximately 14.7 million shares available for future purchase.
Options vesting				4 years																					4 years
Maximum stock options granted per participant				4
Maximum number of restricted stock and RSUs per participant				1
Shares available for issuance				23.8				25.0	45.0							15.0	30.0	30.0	14.7
Maximum shares issued upon exercise of incentive stock options								25
Common stock reserved for future issuances				79.3
ESPP discounted price																			85.00%
Amortization of assumed stock options and RSUs	$ 11,600,000
Stock-based compensation expense	108,300,000	50,910,000	54,002,000																	4,500,000	7,800,000
Total unrecognized compensation expense related to unvested stock options, net of estimated forfeitures																						35,200,000
Total intrinsic value of options exercised																						45,100,000	22,900,000	4,200,000
Cash received from stock option exercises																						80,800,000
Period to recognize total unrecognized compensation expense related to Stock options and Service-based restricted stocks, net of estimated forfeitures on a weighted-average basis													2 years 7 months 6 days									1 year 10 months 24 days
Terms of grant of option																										10 years
Recognition period for unrecognized compensation expense related to performance based restricted stock units net of estimated forfeitures																											3 years
Term of option grants on or after February 12, 2004				7 years
Total unrecognized compensation expense related to unvested Service based awards, net of estimated forfeitures													88,800,000
Weighted-average grant date fair value													69,600,000	55,700,000	40,800,000												25,400,000	26,100,000	16,800,000
Total fair value of shares vested													29,600,000	14,600,000	8,900,000												12,300,000	6,300,000	0
Total unrecognized compensation expense related to performance based awards, net of estimated forfeitures																											12,700,000
Shares of common stock issued under ESPP										6.1	5.8	6.7
Weighted-average price per share										$ 5.09	$ 4.64	$ 4.72
Cash received from ESPP issuances										$ 30,800,000