Fair Value - Additional Information (Detail) (USD $) In Millions, unless otherwise specified																												3 Months Ended	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012		Sep. 30, 2012		Sep. 30, 2013 Fair Value, Inputs, Level 1	Dec. 31, 2012 Fair Value, Inputs, Level 1	Sep. 30, 2012 Fair Value, Inputs, Level 1	Sep. 30, 2013 Fair Value, Inputs, Level 1 Vendor Quoted Pricing	Dec. 31, 2012 Fair Value, Inputs, Level 1 Vendor Quoted Pricing	Sep. 30, 2012 Fair Value, Inputs, Level 1 Vendor Quoted Pricing	Sep. 30, 2013 Fair Value, Inputs, Level 2	Dec. 31, 2012 Fair Value, Inputs, Level 2	Sep. 30, 2012 Fair Value, Inputs, Level 2	Sep. 30, 2013 Fair Value, Inputs, Level 2 Vendor Quoted Pricing	Dec. 31, 2012 Fair Value, Inputs, Level 2 Vendor Quoted Pricing	Sep. 30, 2012 Fair Value, Inputs, Level 2 Vendor Quoted Pricing	Sep. 30, 2013 Fair Value, Inputs, Level 2 Dealer Quoted Pricing	Dec. 31, 2012 Fair Value, Inputs, Level 2 Dealer Quoted Pricing	Sep. 30, 2012 Fair Value, Inputs, Level 2 Dealer Quoted Pricing	Sep. 30, 2013 Fair Value, Inputs, Level 3	Dec. 31, 2012 Fair Value, Inputs, Level 3	Sep. 30, 2012 Fair Value, Inputs, Level 3	Sep. 30, 2013 Fair Value, Inputs, Level 3 Fixed maturities Internal Pricing Investment	Dec. 31, 2012 Fair Value, Inputs, Level 3 Fixed maturities Internal Pricing Investment	Sep. 30, 2012 Fair Value, Inputs, Level 3 Fixed maturities Internal Pricing Investment	Sep. 30, 2013 InvestmentPortfolio [Member] Investment	Sep. 30, 2013 InvestmentPortfolio [Member] Investment	Sep. 30, 2012 Redeemable preferred stocks Fixed maturities Investment	Sep. 30, 2013 Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Dec. 31, 2012 Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Sep. 30, 2012 Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Sep. 30, 2013 Common equities Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Dec. 31, 2012 Common equities Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Sep. 30, 2012 Common equities Fair Value, Inputs, Level 3 Equity securities Internal Pricing Investment	Sep. 30, 2013 Financials Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing	Dec. 31, 2012 Financials Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing	Sep. 30, 2012 Financials Nonredeemable preferred stocks Fair Value, Inputs, Level 3 Equity securities Internal Pricing
Fair Value Measurements [Line Items]
Number of security transferred out of Level 1 into Level 2																												0	0	1
Transferred out of Level 1 into Level 2																														$ 25.0
Quoted prices percentage of fair value assets	100.00%	[1],[2]	100.00%	[1],[2]	100.00%	[1],[2]				59.00%	57.00%	65.00%				97.00%	98.00%	96.00%	3.00%	2.00%	4.00%
Number of securities																									0	0	0				1	1	1	1	1	1
Portfolio	$ 17,995.9	[1],[2]	$ 16,475.5	[1],[2]	$ 17,176.9	[1],[2]	$ 7,040.5	$ 6,723.0	$ 7,557.1				$ 10,849.1	$ 9,637.8	$ 9,505.7							$ 106.3	$ 114.7	$ 114.1							$ 36.1	$ 31.9	$ 29.3	$ 38.0	$ 11.2	$ 11.2	$ 36.1	$ 31.9	$ 29.3

[1] The total fair value of the portfolio at September 30, 2013 and 2012, and December 31, 2012 included $1.3 billion, $1.6 billion, and $1.4 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company.

[2] At September 30, 2013, we had $63.7 million of net unsettled security transactions, including collateral on open derivative positions, recognized in other liabilities, compared to $32.4 million and $90.9 million recognized in other assets at September 30, 2012 and December 31, 2012, respectively.