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Reinsurance - Summary of Effects of Reinsurance on Earned Premiums (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	$ 9,960us-gaap_DirectPremiumsEarned	$ 9,574us-gaap_DirectPremiumsEarned	$ 8,866us-gaap_DirectPremiumsEarned
Earned premiums, Assumed	325us-gaap_AssumedPremiumsEarned	306us-gaap_AssumedPremiumsEarned	244us-gaap_AssumedPremiumsEarned
Earned premiums, Ceded	3,073us-gaap_CededPremiumsEarned	2,609us-gaap_CededPremiumsEarned	2,229us-gaap_CededPremiumsEarned
Earned premiums, Net	7,212us-gaap_PremiumsEarnedNet	7,271us-gaap_PremiumsEarnedNet	6,881us-gaap_PremiumsEarnedNet
Percentage of amount assumed to net	4.50%	4.20%	3.50%
Property and Casualty [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	9,452	9,063	8,354
Earned premiums, Assumed	277	258	197
Earned premiums, Ceded	3,073	2,609	2,229
Earned premiums, Net	6,656	6,712	6,322
Percentage of amount assumed to net	4.20%	3.80%	3.10%
Accident and Health [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Earned premiums, Direct	508	511	512
Earned premiums, Assumed	48	48	47
Earned premiums, Net	$ 556	$ 559	$ 559
Percentage of amount assumed to net	8.60%	8.60%	8.40%

Amount of earned premiums ceded to other entities.

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.