Investments Of Insurance Subsidiaries - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investments Of Insurance Subsidiaries [Line Items]
Investments subject to restrictions	$ 19	$ 19
Available for sale securities expected maturity of debt securities	4 years 7 months 6 days
Available for sale securities average scheduled maturity	8 years 10 months 24 days
Auction Rate Securities [Member]
Investments Of Insurance Subsidiaries [Line Items]
Available for sale securities expected maturity of debt securities	5 years 2 months 12 days
Available for sale securities average scheduled maturity	24 years 4 months 24 days
Asset-Backed Securities [Member]
Investments Of Insurance Subsidiaries [Line Items]
Available for sale securities expected maturity of debt securities	4 years 1 month 6 days
Available for sale securities average scheduled maturity	24 years 1 month 6 days