Investments Investments - Additional Information (Detail) (USD $)	3 Months Ended				6 Months Ended						1 Months Ended	3 Months Ended	6 Months Ended	3 Months Ended	6 Months Ended		3 Months Ended				6 Months Ended		12 Months Ended	6 Months Ended		3 Months Ended		6 Months Ended																	6 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013		Apr. 25, 2014 4.35% Senior Notes due 2044	Jun. 30, 2014 4.35% Senior Notes due 2044	Jun. 30, 2014 4.35% Senior Notes due 2044	Sep. 30, 2013 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Jun. 30, 2014 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067	Jun. 30, 2013 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	Jun. 30, 2014 Interest rate swaps	Dec. 31, 2013 Interest rate swaps	Jun. 30, 2013 Interest rate swaps	Jun. 30, 2014 Interest rate swaps Opened in 2013 Ten Year Time Horizon	Jun. 30, 2013 Interest rate swaps Opened in 2013 Ten Year Time Horizon	Dec. 31, 2013 Interest rate swaps Opened in 2013 Ten Year Time Horizon	Jun. 30, 2013 Interest rate swaps Opened in 2009 Nine Year Time Horizon	Jun. 30, 2013 Interest rate swaps Opened in 2011 Five Year Time Horizon	Jun. 30, 2014 Categories of Investments, Marketable Securities, Trading Securities	Jun. 30, 2013 Categories of Investments, Marketable Securities, Trading Securities	Jun. 30, 2014 Categories of Investments, Marketable Securities, Trading Securities	Jun. 30, 2013 Categories of Investments, Marketable Securities, Trading Securities	Jun. 30, 2014 Fixed Income Securities	Jun. 30, 2014 Equity securities	Dec. 31, 2013 Equity securities	Jun. 30, 2013 Equity securities	Jun. 30, 2014 Equity securities Common equities	Dec. 31, 2013 Equity securities Common equities	Jun. 30, 2013 Equity securities Common equities	Jun. 30, 2014 Equity securities Nonredeemable preferred stocks	Dec. 31, 2013 Equity securities Nonredeemable preferred stocks	Jun. 30, 2013 Equity securities Nonredeemable preferred stocks	Jun. 30, 2014 Equity securities Russell One Thousand Common equities	Jun. 30, 2014 Equity securities Managed Equity Strategy Common equities	Jun. 30, 2014 Fixed maturities	Dec. 31, 2013 Fixed maturities	Jun. 30, 2013 Fixed maturities	Jun. 30, 2014 Fixed maturities State and local government obligations	Dec. 31, 2013 Fixed maturities State and local government obligations	Jun. 30, 2013 Fixed maturities State and local government obligations	Jun. 30, 2014 Fixed maturities Corporate debt securities	Dec. 31, 2013 Fixed maturities Corporate debt securities	Jun. 30, 2013 Fixed maturities Corporate debt securities	Jun. 30, 2014 Interest rate swaps Non-hedging Instruments Fixed maturities Assets		Dec. 31, 2013 Interest rate swaps Non-hedging Instruments Fixed maturities Assets		Jun. 30, 2013 Interest rate swaps Non-hedging Instruments Fixed maturities Assets		Jun. 30, 2013 Interest rate swaps Non-hedging Instruments Other Liabilities Liabilities
Schedule of Investments [Line Items]
Open reverse repurchase commitments	$ 0		$ 416,000,000		$ 0		$ 416,000,000		$ 200,000,000
Realized investment losses																																													800,000			9,500,000
Percentage which the issuer receives the security back at once the change of control is triggered																																																1.00%
Gross unrealized losses	30,600,000		146,700,000		30,600,000		146,700,000		126,000,000																					28,200,000	3,800,000	6,900,000	9,700,000	2,400,000	2,400,000	8,200,000	1,400,000	4,500,000	1,500,000			26,800,000	119,100,000	137,000,000	2,600,000	18,400,000	18,400,000	2,800,000	20,400,000	33,000,000
Percentage of common stock portfolio																																								87.00%	13.00%
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	185		437		185		437		423																						18	31	54	14	24	50	4	7	4			167	392	383	55	141	139	15	51	73
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline																																					1.00%
Additional Expenses Related to the Issuance											700,000	700,000	700,000
Trading securities	0		0		0		0		0
Net realized gains (losses) on securities	40,400,000		134,500,000		159,800,000		215,300,000																			0	0	0	0
Debt Instrument, Issuance Date											Apr. 25, 2014
Debt Instrument, Face Amount											350,000,000
Debt Instrument, Interest Rate, Stated Percentage											4.35%	4.35%	4.35%		6.70%
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)											1,600,000
Repurchase of debt, face amount														54,100,000	0	0
Cash flow hedge gain reclassification from accumulated other comprehensive income to net realized gains/losses on securities																	800,000
Number of derivative positions held during the period																				3	3	3	3	1	2
Derivative Maturity																					10 years	10 years	10 years	9 years	5 years
Balance Sheet - Fair Value	35,100,000	[1]	52,400,000	[1]	35,100,000	[1]	52,400,000	[1]	68,100,000	[1]								35,100,000																																	35,100,000	[1]	68,100,000	[1]	52,400,000	[1]
Notional Value																																																			750,000,000	[2]	750,000,000	[2]	750,000,000	[2]	1,263,000,000	[2]
Cash collateral received from counterparties																		$ 34,100,000	$ 62,700,000	$ 54,400,000

[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 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.