Contracts in Process - Selling, General and Administrative Expenses and Research and Development Expenses (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 26, 2014	Sep. 27, 2013	Sep. 26, 2014	Sep. 27, 2013
Inventory Disclosure [Abstract]
Selling, general and administrative expenses	$ 70	$ 70	$ 213	$ 214
Research and development expenses	14	17	48	62
Total	$ 84	$ 87	$ 261	$ 276

The aggregate costs for the company's commercial businesses, not included included in inventoried contract costs, incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

No definition available.

Name:	lll_ResearchAndDevelopmentExpenses
Namespace Prefix:	lll_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration