Investments Investments - Additional Information (Detail)		3 Months Ended				9 Months Ended
		Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)		Sep. 30, 2016 USD ($)		Sep. 30, 2015 USD ($)		Aug. 25, 2016 USD ($)	Dec. 31, 2015 USD ($)		Jun. 18, 2007
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		68,000,000		0		68,000,000		0			0
Largest Outstanding Balance of Reverse Repurchase Agreements						$ 265,000,000
RRP Largest Outstanding Balance Number of Days Open						1 day
Reverse Repurchase Agreement Average Daily Balance						$ 122,600,000
Gross unrealized losses		$ 45,100,000		$ 127,800,000		$ 45,100,000		$ 127,800,000			$ 166,700,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		554		1,030		554		1,030			1,167
Trading securities		$ 0		$ 0		$ 0		$ 0			$ 0
Net realized gains (losses) on securities		40,900,000		(2,200,000)		90,800,000		116,400,000
Income Statement - Net Realized Gain (Loss) on Securities		700,000		(27,700,000)		(35,800,000)		(29,200,000)
2.45% Senior Notes due 2027
Schedule of Investments [Line Items]
Debt Instrument, Face Amount		$ 500,000,000				$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage		2.45%				2.45%
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%							6.70%
Fixed Income Securities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 40,700,000				$ 40,700,000
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 18,300,000		$ 49,800,000		$ 18,300,000		$ 49,800,000			$ 29,900,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		93		150		93		150			74
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 4,400,000		$ 35,000,000		$ 4,400,000		$ 35,000,000			$ 14,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		86		136		86		136			64
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses		$ 13,900,000		$ 14,800,000		$ 13,900,000		$ 14,800,000			$ 15,700,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		7		14		7		14			10
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline		7.00%				7.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 26,800,000		$ 78,000,000		$ 26,800,000		$ 78,000,000			$ 136,800,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		461		880		461		880			1,093
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						$ 2,100,000
Gross unrealized losses		$ 1,800,000		$ 21,400,000		$ 1,800,000		$ 21,400,000			$ 33,000,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		52		140		52		140			215
Fixed maturities | Redeemable preferred stocks
Schedule of Investments [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities		$ 25,400,000
Gross unrealized losses		$ 1,200,000		$ 22,800,000		$ 1,200,000		$ 22,800,000			$ 43,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions		2		9		2		9			9
Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Collateral Delivered to Counterparties on Derivative Instruments		$ 15,100,000		$ 9,600,000		$ 15,100,000		$ 9,600,000
Cash collateral received from counterparties											$ 4,900,000
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]
Income Statement - Net Realized Gain (Loss) on Securities		$ (1,400,000)		200,000		$ (1,300,000)		200,000
Derivative, Notional Amount		370,000,000	[1]	18,000,000	[1]	370,000,000	[1]	18,000,000	[1]	$ 350,000,000	18,000,000	[1]
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Derivative, Gain (Loss) on Derivative, Net						14,800,000
Not Designated as Hedging Instrument [Member] | Fixed maturities | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Income Statement - Net Realized Gain (Loss) on Securities		0		0		0		0
Derivative, Notional Amount	[1]	0		0		0		0			750,000,000
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Income Statement - Net Realized Gain (Loss) on Securities		300,000		0		(17,000,000)		0
Derivative, Notional Amount	[1]	$ 435,000,000		$ 0		$ 435,000,000		$ 0			$ 0
Derivative, Term of Contract		10 years

[1] 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.