Investments of Insurance Subsidiaries - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investments subject to restrictions	$ 1	$ 9
Available for sale securities expected maturity of debt securities	4 years
Available for sale securities average scheduled maturity	7 years 1 month 6 days
Auction Rate Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available for sale securities expected maturity of debt securities	1 year 4 months 24 days
Available for sale securities average scheduled maturity	22 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	3 years 10 months 24 days
Available for sale securities average scheduled maturity	22 years 8 months 12 days