Consolidated Statements Of Shareholders' Equity (USD $) In Millions, unless otherwise specified	Common Shares [Member]	Common Shares In Treasury [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Deferred Compensation Obligation [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Net Unrealized Appreciation On Investments [Member]	Cumulative Translation Adjustment [Member]	Pension Liability Adjustment [Member]	Common Shares Issued To Employee Trust [Member]	Total
Balance - beginning of period at Dec. 31, 2010	$ 10,161	$ (330)	$ 5,623	$ 5,926	$ 2		$ 1,399	$ 262	$ (67)	$ (2)
Balance- beginning of period, as adjusted at Dec. 31, 2010				5,787
Net shares redeemed under employee share-based compensation plans			(83)
Exercise of stock options	27		11
Dividends declared on Common Shares-par value reduction	(113)
Common Shares issued in treasury, net of net shares redeemed under employee share-based compensation plans		79
Share-based compensation expense and other			34
Net income				250							250
Accumulated other comprehensive income
Change in period, net of income tax expense of $(49) and $14							24
Change in period, net of income tax (expense) of $(32) and $(64)								168
Change in period, net of income tax benefit of $1 and $2									(4)
Balance - end of period at Mar. 31, 2011	10,075	(251)	5,585	6,037	2	1,782	1,423	430	(71)	(2)	23,228
Balance- beginning of period, as adjusted at Dec. 31, 2009
Cumulative effect of adjustment resulting from adoption of new accounting guidance				(139)
Balance - end of period at Dec. 31, 2010				5,926	2					(2)
Balance- beginning of period, as adjusted at Sep. 30, 2010
Cumulative effect of adjustment resulting from adoption of new accounting guidance				(139)
Balance - end of period at Dec. 31, 2010				5,926	2					(2)
Balance - beginning of period at Dec. 31, 2011	10,095	(327)	5,326	7,327			1,715	258	(62)		24,332
Balance- beginning of period, as adjusted at Dec. 31, 2011				7,327
Net shares redeemed under employee share-based compensation plans			(95)
Exercise of stock options			(10)
Common Shares repurchased		(7)
Common Shares issued in treasury, net of net shares redeemed under employee share-based compensation plans		107
Share-based compensation expense and other			33
Funding of dividends declared to Retained earnings			(200)
Net income				973							973
Funding of dividends declared from Additional paid-in capital				200
Dividends declared on Common Shares				(200)
Accumulated other comprehensive income
Change in period, net of income tax expense of $(49) and $14							243
Change in period, net of income tax (expense) of $(32) and $(64)								56
Change in period, net of income tax benefit of $1 and $2									(1)
Balance - end of period at Mar. 31, 2012	$ 10,095	$ (227)	$ 5,054	$ 8,300		$ 2,209	$ 1,958	$ 314	$ (63)		$ 25,431