Marketable Securities - Additional Information (Detail) (United States Government Agencies Corporate Debt Securities And Certificates Of Deposit [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2014
United States Government Agencies Corporate Debt Securities And Certificates Of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, maturing in one year or less	$ 32
Marketable securities, maturing after one year through five years	64
Marketable securities, maturing after five years through ten years	$ 1