Debt Transactions - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended		1 Months Ended				9 Months Ended		1 Months Ended
	Sep. 24, 2012 3.8% Senior Notes due 2022	Sep. 30, 2012 3.8% Senior Notes due 2022	Sep. 30, 2011 3.8% Senior Notes due 2022	Sep. 24, 2012 3.8% Senior Notes due 2022 Insurance Subsidiary	Sep. 24, 2012 3.8% Senior Notes due 2022 Parent Company	Sep. 24, 2012 7 3/8% Senior Notes due in 2013	Sep. 24, 2012 5.875% Junior subordinated debentures due 2052	Sep. 30, 2012 5.875% Junior subordinated debentures due 2052	Sep. 30, 2011 5.875% Junior subordinated debentures due 2052	Oct. 24, 2012 7.1% Trust Preferred Securities Subsequent Event	Oct. 24, 2012 7.1% Junior Subordinated Debt Subsequent Event
Debt Disclosure [Line Items]
Debt Principal Amount	$ 300,000,000						$ 125,000,000
Interest rate	3.80%						5.875%
Periodic interest payments due	Semi-annually						Quarterly
Senior note acquired by subsidiary				150,000,000
Proceeds from issuance of debt	147,000,000	150,000,000	0		297,000,000		121,000,000	125,000,000	0
Future Repayment/Repurchase of Principal Amount						94,000,000
Issuance Cost							4,200,000
Redemption Period							40 years
Preferred Securities redeemed										120,000,000
Redemption of preferred securities and accrued dividend										121,000,000
Repayment of Junior Subordinated Debentures											124,000,000
After tax expected loss on repurchase										$ 2,700,000