Debt and Trust Preferred Securities (Narrative Details) (USD $)			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Senior Notes Due 2021 [Member]	Dec. 31, 2012 Subordinated Debentures Due 2042 [Member]	Dec. 31, 2011 Remarketed on March 4 2011 [Member]
Debt And Trust Preferred Securities [Abstract]
Number Of Trust Preferred Securites					4,500,000
Aggregate Value Of Trust Preferred Securites					$ 158,200,000
Trust Preferred Security Value					$ 35.44
Warrants Excercised		4,402,078			4,400,000
Price Per Warrant		$ 35.44			$ 35.44
Number Of Common Shares Issued					5,500,000
Trust Preferred Securities Fixed Annual Distribution Rate					2.375%
Warrant Conversion Value In Common Shares					1.2508
Loss On Unamortized Issuance Costs					4,400,000
Unexercised Warrant Cash Redemption Value					$ 14.56
Debt Instrument, Interest Rate, Stated Percentage			5.00%	6.20%
Debt Instrument, Maturity Date			Jun. 01, 2021	Sep. 15, 2042
Debt Instrument, Face Amount			400,000,000	400,000,000
Proceeds from Issuance of Senior Long-term Debt			394,400,000
Debt Issuance Costs Incurred And Capitalized			3,400,000	6,300,000
ProceedsFromIssuanceOfSubordinatedLongTermDebt				393,700,000
Line of Credit Facility [Line Items]
Syndicated Revolving Credit Facility Capacity	850,000,000
Syndicated Revolving Credit Facility Capacity Undrawn Outstanding	$ 402,900,000	$ 183,500,000
Debt, Weighted Average Interest Rate	5.99%	5.94%