Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended						3 Months Ended		9 Months Ended		12 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended		6 Months Ended				6 Months Ended		3 Months Ended				9 Months Ended									9 Months Ended					9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012		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	Dec. 31, 2012 Corporate Credit Default Swaps Financial Services Sector	Sep. 30, 2012 Corporate Credit Default Swaps Financial Services Sector	Sep. 30, 2013 Corporate Credit Default Swaps Financial Services Sector	Sep. 30, 2012 Corporate Credit Default Swaps Financial Services Sector	Jun. 30, 2013 Interest rate swaps	Sep. 30, 2013 Interest rate swaps	Dec. 31, 2012 Interest rate swaps	Sep. 30, 2012 Interest rate swaps	Jun. 30, 2013 Interest rate swaps Opened in 2013 Ten Year Time Horizon	Sep. 30, 2013 Interest rate swaps Opened in 2013 Ten Year Time Horizon	Sep. 30, 2013 Interest rate swaps Opened in 2009 Closed Positions Nine Year Time Horizon	Sep. 30, 2013 Interest rate swaps Opened in 2011 Closed Positions Five Year Time Horizon	Sep. 30, 2013 Categories of Investments, Marketable Securities, Trading Securities	Sep. 30, 2012 Categories of Investments, Marketable Securities, Trading Securities	Sep. 30, 2013 Categories of Investments, Marketable Securities, Trading Securities	Sep. 30, 2012 Categories of Investments, Marketable Securities, Trading Securities	Sep. 30, 2013 Fixed Income Securities	Sep. 30, 2013 Equity securities	Dec. 31, 2012 Equity securities	Sep. 30, 2012 Equity securities	Sep. 30, 2013 Equity securities Common equities	Dec. 31, 2012 Equity securities Common equities	Sep. 30, 2012 Equity securities Common equities	Sep. 30, 2013 Equity securities Russell One Thousand Common equities	Sep. 30, 2013 Equity securities Managed Equity Strategy Common equities	Sep. 30, 2013 Fixed maturities	Dec. 31, 2012 Fixed maturities	Sep. 30, 2012 Fixed maturities	Sep. 30, 2013 Fixed maturities Corporate debt securities	Dec. 31, 2012 Fixed maturities Corporate debt securities	Sep. 30, 2012 Fixed maturities Corporate debt securities	Sep. 30, 2013 Interest rate swaps Non-hedging Instruments Closed Positions		Dec. 31, 2012 Interest rate swaps Non-hedging Instruments Closed Positions		Sep. 30, 2012 Interest rate swaps Non-hedging Instruments Closed Positions		Sep. 30, 2013 Interest rate swaps Non-hedging Instruments Other Liabilities Liabilities		Dec. 31, 2012 Interest rate swaps Non-hedging Instruments Other Liabilities Liabilities		Sep. 30, 2012 Interest rate swaps Non-hedging Instruments Other Liabilities Liabilities		Sep. 30, 2013 Interest rate swaps Non-hedging Instruments Fixed maturities Assets		Dec. 31, 2012 Interest rate swaps Non-hedging Instruments Fixed maturities Assets		Sep. 30, 2012 Interest rate swaps Non-hedging Instruments Fixed maturities Assets
Schedule of Investments [Line Items]
Open reverse repurchase commitments	$ 278.1		$ 798.1		$ 278.1		$ 798.1		$ 581.0
Percentage which the issuer receives the security back at once the change of control is triggered																																																	1.00%
Realized investment losses																																																	12.3
Gross unrealized losses	129.0		35.3		129.0		35.3		34.0																												124.0	9.8	10.3	6.1	5.0	10.3	6.1			119.2	23.7	29.2	28.9	1.0	0.4
Percentage of common stock portfolio																																												88.00%	12.00%
Trading securities	0		0		0		0		0
Net realized gains (losses) on securities	29.8		172.3		245.1		251.4																										0	0	0	0
Repurchase of debt, face amount											54.1	0.5	54.1	30.9	30.9
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
Number of derivative positions opened during the period																													3
Derivative maturity, term																						5 years		5 years						10 years	9 years	5 years
Derivative Assets (Liabilities), at Fair Value, Net	(53.5)	[1]	101.3	[1]	(53.5)	[1]	101.3	[1]	95.5	[1]																(53.5)																										0	[1]	0	[1]	0	[1]	0	[1]	95.5	[1]	101.6	[1]	(53.5)	[1]	0	[1]	0	[1]
Notional Value																																																				1,263.0	[2]	0	[2]	0	[2]	0	[2]	1,263.0	[2]	1,263.0	[2]	750.0	[2]	0	[2]	0	[2]
Number of derivative positions closed during the period																									3						1	2
Cash collateral delivered to the counterparties																											105.0	107.2
Number of derivative positions held during the period																					0	1	0	1
Cash collateral received from counterparties																						$ 0.3		$ 0.3		$ 49.3

[1] To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we expect that we will report them on a gross basis on our balance sheets, consistent with our historical presentation.

[2] The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions.