Debt - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended			1 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 7% Notes due 2013	Oct. 31, 2013 7% Notes due 2013 Notes scheduled to be paid in 2013 Subsequent Event [Member]	Sep. 30, 2013 3.75% Senior Notes due 2021	Sep. 30, 2013 6 5/8% Senior Notes due 2029	Sep. 30, 2013 6.25% Senior Notes due 2032	Sep. 30, 2013 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Sep. 30, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Sep. 30, 2013 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Sep. 30, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Dec. 31, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Sep. 30, 2013 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 Net unrealized gains on forecasted transactions	Sep. 30, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 Net unrealized gains on forecasted transactions	Sep. 30, 2013 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 Net unrealized gains on forecasted transactions	Sep. 30, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 Net unrealized gains on forecasted transactions	Dec. 31, 2012 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 Net unrealized gains on forecasted transactions
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage					7.00%		3.75%	6.625%	6.25%	6.70%		6.70%
Repurchase of debt, face amount										$ 54.1	$ 0.5	$ 54.1	$ 30.9	$ 30.9
(Gains) losses on extinguishment of debt	4.3	0.1	4.3	1.8						4.3	0.1	4.3	1.8	1.8
Cash flow hedge gain reclassification from accumulated other comprehensive income to net realized gains/losses on securities															0.8	0	0.8	0.6	0.6
Payment of debt			$ 0	$ 350.0		$ 150.0