Summary of Amortized Cost and Principal Amounts of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Book value	$ 790	$ 790
Principal amount	793	793
Senior Debentures 6.900 Percent Due 2028
Debt Instrument [Line Items]
Interest rate	6.90%	6.90%
Debt issuance date	1998	1998
Book value	28	28
Principal amount	28	28
Senior Debentures 6.920 Percent Due 2028
Debt Instrument [Line Items]
Interest rate	6.92%	6.92%
Debt issuance date	2005	2005
Book value	391	391
Principal amount	391	391
Senior Notes 6.125 Percent Due 2034
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%
Debt issuance date	2004	2004
Book value	371	371
Principal amount	$ 374	$ 374