Shareholders' Equity (Details) (USD $)	3 Months Ended	12 Months Ended																0 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 [CommonStockMember]	Dec. 31, 2011 [CommonStockMember]	Dec. 31, 2010 [CommonStockMember]	Dec. 31, 2012 [SeriesCPreferredStockMember]	Dec. 31, 2011 [SeriesCPreferredStockMember]	Dec. 31, 2012 [SeriesDPreferredStockMember]	Dec. 31, 2011 [SeriesDPreferredStockMember]	Dec. 31, 2012 [SeriesEPreferredStockMember]	Dec. 31, 2011 [SeriesEPreferredStockMember]	Dec. 31, 2012 [PreferredSharesDesignatedAndRedeemedMember]	Dec. 31, 2011 [PreferredSharesDesignatedAndRedeemedMember]	Dec. 31, 2012 [UndesignatedSharesMember]	Dec. 31, 2011 [UndesignatedSharesMember]	Feb. 11, 2013 [SeriesFPreferredStockMember]
Authorized Shares [Line Items]
Total authorized shares		200,000,000	200,000,000		130,000,000	130,000,000		11,600,000	11,600,000	9,200,000	9,200,000	15,000,000	15,000,000	14,000,000	14,000,000	20,200,000	20,200,000
Par value per share	$ 1		$ 1
Common Shares [Abstract]
Shares repurchased					7,100,000	5,400,000	14,000,000
Shares held in treasury	26,550,530	26,550,530	19,444,365
Repurchase of common shares					$ 532,900,000	$ 396,200,000	$ 1,082,600,000
Average cost per share of shares repurchased					$ 75.00	$ 73.41	$ 77.1
Current share repurchase authorization outstanding					3,200,000
Preferred Shares [Abstract]
Preferred shares issuance, date								May 08, 2003		Nov. 09, 2004		Jun. 08, 2011						Feb. 11, 2013
Preferred shares, shares issued	35,750,000	35,750,000	35,750,000					11,600,000		9,200,000		15,000,000						10,000,000
Dividend rate, percentage								6.75%		6.50%		7.25%						5.875%
Distribution on liquidation								$ 25.00		$ 25.00		$ 25.00						$ 25
Aggregate liquidation value	893,750,000	893,750,000	893,750,000					290,000,000		230,000,000		373,800,000						250,000,000
Net Proceeds From Issuance Of Preferred Stock And Preference Stock		0	361,722,000	0				280,900,000		222,300,000		361,700,000						242,300,000
Underwriting discounts and commissions								$ 9,100,000		$ 7,700,000		$ 12,100,000						$ 7,700,000