Shareholders' Equity (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Authorized Shares [Line Items]
Total authorized shares	200,000,000	200,000,000
Par value per share	$ 1	$ 1
Common Shares [Abstract]
Shares held in treasury	19,444,365	14,046,895
Preferred Shares [Abstract]
Preferred shares, shares issued	35,750,000	20,800,000
Aggregate liquidation value	$ 893,750,000	$ 520,000,000
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	361,722,000	0	0
[CommonStockMember]
Authorized Shares [Line Items]
Total authorized shares	130,000,000	130,000,000
Common Shares [Abstract]
Shares issued			25,700,000
Shares reissued from treasury			1,300,000
Shares repurchased	5,400,000	14,000,000	0
Repurchase of common shares	396,200,000	1,082,600,000
Average cost per share of shares repurchased	$ 73.41	$ 77.10
Current share repurchase authorization outstanding	5,300,000
Face value of extended forward agreement settlement		200,000,000
[SeriesCPreferredStockMember]
Authorized Shares [Line Items]
Total authorized shares	11,600,000	11,600,000
Preferred Shares [Abstract]
Preferred shares issuance, date	May 08, 2003
Preferred shares, shares issued	11,600,000
Dividend rate, percentage	6.75%
Distribution on liquidation	$ 25.00
Aggregate liquidation value	290,000,000
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	280,900,000
Underwriting discounts and commissions	9,100,000
[SeriesDPreferredStockMember]
Authorized Shares [Line Items]
Total authorized shares	9,200,000	9,200,000
Preferred Shares [Abstract]
Preferred shares issuance, date	Nov. 09, 2004
Preferred shares, shares issued	9,200,000
Dividend rate, percentage	6.50%
Distribution on liquidation	$ 25.00
Aggregate liquidation value	230,000,000
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	222,300,000
Underwriting discounts and commissions	7,700,000
[SeriesEPreferredStockMember]
Authorized Shares [Line Items]
Total authorized shares	15,000,000	0
Preferred Shares [Abstract]
Preferred shares issuance, date	Jun. 08, 2011
Preferred shares, shares issued	15,000,000
Dividend rate, percentage	7.25%
Distribution on liquidation	$ 25.00
Aggregate liquidation value	373,800,000
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	361,700,000
Underwriting discounts and commissions	12,100,000
Gross Proceeds from issuance of preferred shares	$ 374,000,000
[PreferredSharesDesignatedAndRedeemedMember]
Authorized Shares [Line Items]
Total authorized shares	14,000,000	14,000,000
[UndesignatedSharesMember]
Authorized Shares [Line Items]
Total authorized shares	20,200,000	35,200,000