Stock-Based Compensation - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended		12 Months Ended
	Dec. 31, 2012 Securities	Dec. 31, 2012 Maximum [Member]	Dec. 31, 2012 Minimum [Member]	Jul. 31, 2012 2009 Employee Stock Purchase Plan [Member]	Jan. 31, 2012 2009 Employee Stock Purchase Plan [Member]	Dec. 31, 2012 2009 Employee Stock Purchase Plan [Member]	Dec. 31, 2012 2008 Long Term Performance Plan [Member]	Dec. 31, 2012 Stock Options [Member]	Dec. 31, 2011 Stock Options [Member]	Dec. 31, 2010 Stock Options [Member]	Dec. 31, 2012 Restricted Stock Units [Member] Age	Dec. 31, 2011 Restricted Stock Units [Member]	Dec. 31, 2010 Restricted Stock Units [Member]	Dec. 31, 2012 Performance Units [Member]	Dec. 31, 2011 Performance Units [Member]	Dec. 31, 2010 Performance Units [Member]	Dec. 31, 2012 2008 Directors Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum number of shares of L-3 Holdings common stock that may be issued pursuant to "incentive" stock option awards							3,131,034
Maximum number of shares of L-3 Holdings common stock that may be issued pursuant to all awards granted during a calendar year to an individual							521,839
Percentage of total outstanding shares of common stock that may be issued in respect to performance based awards							5.00%
Maximum number of shares authorized for issuance under the amendment						5,100,000	12,200,000										312,995
Maximum number of shares of L-3 Holdings common stock may be issued pursuant to "full value" awards under the amendment							3,700,000
Shares of L-3 Holdings common stock available to be awarded	2.6						265,552
Maximum number of shares authorized for issuance under the amendment increased due to spin-off of Engility							12,700,000
Conversion ratio for stock options, restricted stock, performance units	1.043678
Expiration period of stock options granted from date of grant								10 years
Vesting period								3 years			3 years
Weighted average grant date fair value of stock options								$ 11.32	$ 15.54	$ 18.41
Intrinsic value of stock options exercised								$ 7,000,000	$ 8,000,000	$ 30,000,000
Unrecognized compensation costs related to equity awards after income taxes								10,000,000			27,000,000			4,000,000
Unrecognized compensation costs related to stock awards								6,000,000			43,000,000			6,000,000
Weighted average remaining period for recognition of unrecognized costs related to equity awards								1 year 1 month 6 days			1 year 2 months 12 days			1 year 3 months 18 days
Actual income tax benefit realized related to compensation deductions arising from the exercise of stock options								2,000,000	3,000,000	10,000,000
Vesting period of units granted for non-employee directors											1 year
Weighted average grant date fair value	$ 70.42										$ 70.42	$ 80.17	$ 90.23	$ 70.43	$ 95.50	$ 105.14
Length of service after grant date											1 year			3 years	3 years
Age of retirement eligible employees											65
Length of service for retirement eligible employees, Minimum											At least five years
Fair value of restricted stock unit awards vested											44,000,000	35,000,000	31,000,000
Minimum payout range														0.00%
Maximum payout range														200.00%
Total performance units earned														53,229
Fair market value of total performance units earned as of their vesting date														4,100,000
Percentage of fair market value for offering periods						95.00%
Term of plans						Eligible employees are offered options to purchase shares of L-3 Holdings' common stock at the end of each six-month offering period at 95% of fair market value (or 85% of fair market value for offering periods beginning prior to January 1, 2011) based on the average of the highest and lowest sales prices for the stock on the purchase date.
Offering period to purchase shares						6 months
Share purchases funded through payroll deductions up to a maximum percentage						10.00%
Share purchases funded through payroll deductions up to a maximum value						$ 21,250
Issued shares under the ESPP, shares				300,000	300,000
Issued shares under the ESPP, value				$ 69.61	$ 71.87
Employee discount on the purchase of common stock		15.00%	5.00%