Fair Value Measurements - Summary of Book Value and Principal Amounts of Long-Term Debt (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Book value	$ 790	$ 790
Principal amount of debt	793	793
Senior Debentures 6.900% Due 2028
Debt Instrument [Line Items]
Interest rate	6.90%	6.90%
Debt issuance year	1998	1998
Book value	28	28
Principal amount of debt	28	28
Senior Debentures 6.920% Due 2028
Debt Instrument [Line Items]
Interest rate	6.92%	6.92%
Debt issuance year	2005	2005
Book value	391	391
Principal amount of debt	391	391
Senior Notes 6.152% Due 2034
Debt Instrument [Line Items]
Interest rate	6.125%	6.125%
Debt issuance year	2004	2004
Book value	371	371
Principal amount of debt	$ 374	$ 374