Investments - Additional Information (Detail)		3 Months Ended			9 Months Ended
	Apr. 03, 2017 USD ($)	Sep. 30, 2017 USD ($) security	Jun. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) security	Sep. 30, 2017 USD ($) security	Sep. 30, 2016 USD ($) security	Dec. 31, 2016 USD ($) security	Aug. 25, 2016 USD ($)
Schedule of Investments [Line Items]
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase					$ 0	$ 0
Securities Sold under Agreements to Repurchase		$ 0		$ 0	0	0	$ 0
Open reverse repurchase commitments		0		68,000,000	0	68,000,000	0
Proceeds from Securities Purchased under Agreements to Resell					0
Largest Outstanding Balance of Reverse Repurchase Agreements						$ 265,000,000
RRP Largest Outstanding Balance Number of Days Open						1 day
Reverse Repurchase Agreements, Total Days of Activity						28 days
Reverse Repurchase Agreement Average Daily Balance						$ 122,600,000
Gross unrealized losses		$ 60,700,000		$ 45,100,000	$ 60,700,000	$ 45,100,000	$ 139,800,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		852		554	852	554	1,369
Trading securities		$ 0		$ 0	$ 0	$ 0	$ 0
Net realized gains (losses) on securities		18,300,000		40,900,000	117,100,000	90,800,000
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(700,000)	0	35,800,000
Balance Sheet - Fair Value		$ 0		(13,000,000)	$ 0	(13,000,000)	0
4.125% Senior Notes due 2047
Schedule of Investments [Line Items]
Debt, face amount	$ 850,000,000		$ 850,000,000
Debt, stated interest rate	4.125%	4.125%	4.125%		4.125%
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)		$ (8,000,000)
2.45% Senior Notes due 2027
Schedule of Investments [Line Items]
Debt, face amount				$ 500,000,000		$ 500,000,000
Debt, stated interest rate		2.45%		2.45%	2.45%	2.45%
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Schedule of Investments [Line Items]
Debt, stated interest rate		6.70%	6.70%		6.70%
Debt Instrument, Repurchased Face Amount		$ 31,000,000		$ 20,000,000	$ 31,000,000	$ 20,000,000
Fixed Income Securities
Schedule of Investments [Line Items]
Gross unrealized losses		55,200,000			55,200,000
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 12,800,000		$ 18,300,000	$ 12,800,000	$ 18,300,000	$ 18,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		70		93	70	93	88
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 5,500,000		$ 4,400,000	$ 5,500,000	$ 4,400,000	$ 2,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		67		86	67	86	75
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses		$ 7,300,000		$ 13,900,000	$ 7,300,000	$ 13,900,000	$ 16,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		3		7	3	7	13
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline		9.00%			9.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 47,900,000		$ 26,800,000	$ 47,900,000	$ 26,800,000	$ 121,600,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		782		461	782	461	1,281
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					$ 9,600,000
Gross unrealized losses		$ 4,200,000		$ 1,800,000	$ 4,200,000	$ 1,800,000	$ 24,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		113		52	113	52	184
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Net realized gains (losses) on securities		$ 0		$ 0	$ 0	$ 0
Russell One Thousand | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio		95.00%			95.00%
Managed Equity Strategy | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio		5.00%			5.00%
Designated as Hedging Instruments | Ineffective Cash Flow Hedge [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		$ 0		1,400,000	$ 0	1,300,000
Derivative, Notional Amount		31,000,000		370,000,000	31,000,000	370,000,000	$ 370,000,000	$ 350,000,000
Balance Sheet - Fair Value		0		0	0	0	0
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value		0			0		0
Derivative, Collateral, Right to Reclaim Cash				15,100,000		15,100,000
Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value		0		0	0	0	0
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(300,000)	0	17,000,000
Derivative, Notional Amount		0		435,000,000	0	435,000,000	750,000,000
Balance Sheet - Fair Value		0		0	0	0	0
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(100,000)	0	(300,000)
Derivative, Notional Amount		0		135,000,000	0	135,000,000	135,000,000
Balance Sheet - Fair Value		$ 0		$ 0	$ 0	$ 0	$ 0