Quarterly Financial Data - Summary of Quarterly Financial Data (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 3,476	$ 3,574	$ 3,542	$ 3,506	$ 3,476	$ 3,403	$ 3,551	$ 3,517	$ 14,098	$ 13,947	$ 13,595
Losses and expenses	2,714	2,756	2,864	2,903	2,690	2,664	2,755	2,601
Federal and Foreign Income Tax	204	224	179	154	217	198	217	260	761	892	451
NET INCOME	$ 558	$ 594	$ 499	$ 449	$ 569	$ 541	$ 579	$ 656	$ 2,100	$ 2,345	$ 1,545
Basic earnings per share	$ 2.35	$ 2.47	$ 2.03	$ 1.81	$ 2.25	$ 2.11	$ 2.22	$ 2.49	$ 8.65	$ 9.08	$ 5.73
Diluted earnings per share	$ 2.35	$ 2.47	$ 2.03	$ 1.80	$ 2.24	$ 2.10	$ 2.21	$ 2.48	$ 8.62	$ 9.04	$ 5.69
Underwriting ratios
Losses to premiums earned	53.8	54.0	58.7	61.1	54.7	53.0	56.7	52.3
Expenses to premiums written	30.5	31.8	31.3	32.1	30.8	32.7	32.1	32.3
Combined	84.3	85.8	90.0	93.2	85.5	85.7	88.8	84.6