Reinsurance - Effect of Reinsurance on Premiums Written and Earned (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Direct premiums written	$ 12,647	$ 12,452	$ 12,072
Reinsurance assumed	423	398	271
Reinsurance ceded	(1,200)	(1,092)	(1,107)
Net premiums written	11,870	11,758	11,236
Direct premiums earned	12,596	12,387	12,059
Reinsurance assumed	422	365	253
Reinsurance ceded	(1,180)	(1,108)	(1,097)
Net premiums earned	$ 11,838	$ 11,644	$ 11,215