Investments Investments - Additional Information (Detail)		1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
		Jan. 26, 2015 USD ($)	Apr. 25, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Apr. 01, 2015	Jun. 18, 2007
Schedule of Investments [Line Items]
Open reverse repurchase commitments				$ 0	$ 0	$ 0	$ 0
Gross unrealized losses				$ 104,800,000	$ 30,600,000	$ 104,800,000	$ 30,600,000	$ 49,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				1,385	185	1,385	185	318
Trading securities				$ 0	$ 0	$ 0	$ 0	$ 0
Net realized gains (losses) on securities				77,700,000	40,400,000	118,600,000	159,800,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest				368,500,000	293,400,000	664,100,000	614,700,000
Balance Sheet - Fair Value	[1]			$ 18,300,000	35,100,000	18,300,000	35,100,000	15,800,000
Largest Outstanding Balance of Reverse Repurchase Agreements						$ 275,000,000
RRP Largest Outstanding Balance Number of Days Open						1 day
Reverse Repurchase Agreement Average Daily Balance						$ 135,800,000
3.70% Senior Notes due 2045
Schedule of Investments [Line Items]
Additional Expenses Related to the Issuance		$ 800,000
Debt Instrument, Issuance Date		Jan. 26, 2015
Debt Instrument, Face Amount		$ 400,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		3.70%		3.70%		3.70%				6.70%
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)		$ 12,900,000
4.35% Senior Notes due 2044
Schedule of Investments [Line Items]
Additional Expenses Related to the Issuance			$ 700,000
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%
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)			$ 1,600,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Schedule of Investments [Line Items]
Debt Instrument, Interest Rate, Stated Percentage				6.70%		6.70%
Repurchase of debt, face amount						$ 0	0	44,300,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 | Net unrealized gains on forecasted transactions
Schedule of Investments [Line Items]
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)								(500,000)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest						0	0
Future [Member]
Schedule of Investments [Line Items]
Derivative, Gain (Loss) on Derivative, Net						1,800,000
Collateral Delivered to Counterparties on Derivative Instruments				$ 1,200,000		1,200,000
Fixed Income Securities
Schedule of Investments [Line Items]
Gross unrealized losses				95,200,000		95,200,000
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses				$ 20,900,000	$ 3,800,000	$ 20,900,000	$ 3,800,000	$ 16,500,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				87	18	87	18	28
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses				$ 9,600,000	$ 2,400,000	$ 9,600,000	$ 2,400,000	$ 10,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				73	14	73	14	20
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses				$ 11,300,000	$ 1,400,000	$ 11,300,000	$ 1,400,000	$ 6,400,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				14	4	14	4	8
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline				3.00%		3.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses				$ 83,900,000	$ 26,800,000	$ 83,900,000	$ 26,800,000	$ 32,700,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				1,298	167	1,298	167	290
Fixed maturities | State and local government obligations
Schedule of Investments [Line Items]
Gross unrealized losses				$ 17,100,000	$ 2,600,000	$ 17,100,000	$ 2,600,000	$ 1,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				736	55	736	55	46
Fixed maturities | Corporate debt securities
Schedule of Investments [Line Items]
Percentage which the issuer receives the security back at once the change of control is triggered				1.00%		1.00%
Realized investment losses						$ 5,100,000
Gross unrealized losses				$ 22,600,000	$ 2,800,000	$ 22,600,000	$ 2,800,000	$ 10,400,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				160	15	160	15	53
Interest rate swaps
Schedule of Investments [Line Items]
Derivative, Gain (Loss) on Derivative, Net						$ 2,800,000
Balance Sheet - Fair Value				$ 18,600,000		18,600,000
Cash collateral received from counterparties				$ 21,900,000	$ 34,100,000	21,900,000	$ 34,100,000	$ 16,100,000
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Net realized gains (losses) on securities					0	$ 0	0
Russell One Thousand | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio				88.00%		88.00%
Managed Equity Strategy | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio				12.00%		12.00%
Not Designated as Hedging Instrument [Member] | Future [Member] | Closed Positions
Schedule of Investments [Line Items]
Notional Value	[2]			$ 326,000,000	0	$ 326,000,000	0	0
Balance Sheet - Fair Value	[1]			0	0	0	0	0
Not Designated as Hedging Instrument [Member] | Future [Member] | Fixed maturities | Assets
Schedule of Investments [Line Items]
Notional Value	[2]				0		0
Not Designated as Hedging Instrument [Member] | Interest rate swaps | Fixed maturities | Assets
Schedule of Investments [Line Items]
Notional Value	[2]			750,000,000	750,000,000	750,000,000	750,000,000	750,000,000
Balance Sheet - Fair Value	[1]			18,600,000	35,100,000	18,600,000	35,100,000	15,800,000
Designated as Hedging Instruments | Ineffective Cash Flow Hedge | Closed Positions
Schedule of Investments [Line Items]
Notional Value	[2]			0	0	0	0	44,000,000
Balance Sheet - Fair Value	[1]			$ 0	$ 0	$ 0	$ 0	$ 0
ARX Holding Corp. [Member]
Schedule of Investments [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions				806		806
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline				1.00%		1.00%
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest				$ 16,800,000
Ownership interest held prior to acquisition (percent)									5.00%

[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.