Investments of Insurance Subsidiaries - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments subject to restrictions	$ 9	$ 19
Available for sale securities expected maturity of debt securities	4 years 6 months
Available for sale securities average scheduled maturity	8 years 3 months 18 days
Auction rate securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available for sale securities expected maturity of debt securities	5 years 6 months
Available for sale securities average scheduled maturity	24 years 1 month 6 days
Asset-backed securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available for sale securities expected maturity of debt securities	4 years
Available for sale securities average scheduled maturity	24 years