Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Fair Value Measurements [Line Items]
Total investments at fair value	[1],[2]	$ 20,577.4	$ 19,018.0	$ 18,759.2
Long-Term Debt, Fair Value		2,942.4	2,527.5	2,551.6
Debt		2,739.0	2,164.7	2,208.0
Fixed maturities
Fair Value Measurements [Line Items]
Total investments at fair value		15,589.0	13,549.2	12,498.6
Fixed maturities | Redeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		271.7	279.2	287.5
Fixed maturities | Redeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		97.5	97.9	103.4
Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value Measurements [Line Items]
Total investments at fair value		61.2	65.3	65.3
Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value Measurements [Line Items]
Total investments at fair value		113.0	116.0	118.8
Fixed maturities | Debt Securities
Fair Value Measurements [Line Items]
Total investments at fair value		8,708.9	7,657.2	7,883.4
Fixed maturities | Debt Securities | U.S. government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		2,063.2	2,667.1	3,289.4
Fixed maturities | Debt Securities | State and local government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		3,192.3	2,139.2	2,331.2
Fixed maturities | Debt Securities | Foreign government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		18.6	14.2	0.0
Fixed maturities | Debt Securities | Corporate debt securities
Fair Value Measurements [Line Items]
Total investments at fair value		3,434.8	2,836.7	2,262.8
Fixed maturities | Asset-backed Securities
Fair Value Measurements [Line Items]
Total investments at fair value		6,608.4	5,612.8	4,327.7
Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		1,892.5	1,658.5	1,335.4
Fixed maturities | Asset-backed Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements [Line Items]
Total investments at fair value		114.3	0.0	0.0
Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,560.1	2,315.6	2,019.3
Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,041.5	1,638.7	973.0
Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		772.3	827.5	760.2
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		772.3	827.5	760.2
Equity securities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		2,546.8	2,492.3	2,381.7
Equity securities | Common equities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		2,546.5	2,491.9	2,381.2
Equity securities | Common equities | Other risk investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.3	0.4	0.5
Total Fixed Maturities and Equity Securities
Fair Value Measurements [Line Items]
Total investments at fair value		18,908.1	16,869.0	15,640.5
Short-term investments
Fair Value Measurements [Line Items]
Total investments at fair value		1,669.3	2,149.0	3,118.7
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Total investments at fair value		6,230.2	7,314.3	8,752.6
Long-Term Debt, Fair Value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities
Fair Value Measurements [Line Items]
Total investments at fair value		2,081.8	2,681.3	3,289.4
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,081.8	2,681.3	3,289.4
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities | U.S. government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		2,063.2	2,667.1	3,289.4
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities | State and local government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities | Foreign government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		18.6	14.2	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities | Corporate debt securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		167.7	204.1	238.7
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		167.7	204.1	238.7
Fair Value, Inputs, Level 1 | Equity securities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		2,546.5	2,491.9	2,381.2
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		2,546.5	2,491.9	2,381.2
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Other risk investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total Fixed Maturities and Equity Securities
Fair Value Measurements [Line Items]
Total investments at fair value		4,796.0	5,377.3	5,909.3
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value Measurements [Line Items]
Total investments at fair value		1,434.2	1,937.0	2,843.3
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Total investments at fair value		14,335.0	11,622.4	9,934.7
Long-Term Debt, Fair Value		2,763.9	2,527.5	2,551.6
Fair Value, Inputs, Level 2 | Fixed maturities
Fair Value Measurements [Line Items]
Total investments at fair value		13,495.3	10,856.3	9,181.6
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		271.7	279.2	287.5
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		97.5	97.9	103.4
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value Measurements [Line Items]
Total investments at fair value		61.2	65.3	65.3
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value Measurements [Line Items]
Total investments at fair value		113.0	116.0	118.8
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities
Fair Value Measurements [Line Items]
Total investments at fair value		6,626.1	4,975.9	4,594.0
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities | U.S. government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities | State and local government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		3,192.3	2,139.2	2,331.2
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities | Foreign government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities | Corporate debt securities
Fair Value Measurements [Line Items]
Total investments at fair value		3,433.8	2,836.7	2,262.8
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities
Fair Value Measurements [Line Items]
Total investments at fair value		6,597.5	5,601.2	4,300.1
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		1,892.5	1,658.5	1,335.4
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements [Line Items]
Total investments at fair value		114.3	0.0	0.0
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,549.2	2,304.0	1,991.7
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,041.5	1,638.7	973.0
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		604.6	554.1	477.7
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		604.6	554.1	477.7
Fair Value, Inputs, Level 2 | Equity securities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Other risk investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Total Fixed Maturities and Equity Securities
Fair Value Measurements [Line Items]
Total investments at fair value		14,099.9	11,410.4	9,659.3
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value Measurements [Line Items]
Total investments at fair value		235.1	212.0	275.4
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Total investments at fair value		12.2	81.3	71.9
Long-Term Debt, Fair Value		178.5	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value Measurements [Line Items]
Total investments at fair value		11.9	11.6	27.6
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities
Fair Value Measurements [Line Items]
Total investments at fair value		1.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities | U.S. government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities | State and local government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities | Foreign government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities | Corporate debt securities
Fair Value Measurements [Line Items]
Total investments at fair value		1.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities
Fair Value Measurements [Line Items]
Total investments at fair value		10.9	11.6	27.6
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		10.9	11.6	27.6
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	69.3	43.8
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	69.3	43.8
Fair Value, Inputs, Level 3 | Equity securities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		0.3	0.4	0.5
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Other risk investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.3	0.4	0.5
Fair Value, Inputs, Level 3 | Total Fixed Maturities and Equity Securities
Fair Value Measurements [Line Items]
Total investments at fair value		12.2	81.3	71.9
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.0	0.0	0.0
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value Measurements [Line Items]
Total investments at fair value		19,174.0	17,402.8	17,146.4
Debt		2,739.0	2,164.7	2,208.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities
Fair Value Measurements [Line Items]
Total investments at fair value		15,520.6	13,374.2	12,265.4
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		259.8	260.2	262.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		76.8	77.3	79.2
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value Measurements [Line Items]
Total investments at fair value		65.0	65.0	65.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value Measurements [Line Items]
Total investments at fair value		118.0	117.9	117.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities
Fair Value Measurements [Line Items]
Total investments at fair value		8,679.0	7,564.9	7,751.6
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities | U.S. government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		2,049.3	2,641.1	3,249.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities | State and local government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		3,177.2	2,095.7	2,287.2
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities | Foreign government obligations
Fair Value Measurements [Line Items]
Total investments at fair value		18.6	14.2	0.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities | Corporate debt securities
Fair Value Measurements [Line Items]
Total investments at fair value		3,433.9	2,813.9	2,215.4
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities
Fair Value Measurements [Line Items]
Total investments at fair value		6,581.8	5,549.1	4,251.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		1,879.5	1,635.5	1,312.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value Measurements [Line Items]
Total investments at fair value		116.0	0.0	0.0
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,548.4	2,278.7	1,974.4
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value Measurements [Line Items]
Total investments at fair value		2,037.9	1,634.9	965.4
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Total investments at fair value		632.4	590.4	497.1
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Measurements [Line Items]
Total investments at fair value		632.4	590.4	497.1
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		1,351.7	1,289.2	1,265.2
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Common equities
Fair Value Measurements [Line Items]
Total investments at fair value		1,351.4	1,288.8	1,264.7
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Other risk investments
Fair Value Measurements [Line Items]
Total investments at fair value		0.3	0.4	0.5
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities and Equity Securities
Fair Value Measurements [Line Items]
Total investments at fair value		17,504.7	15,253.8	14,027.7
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value Measurements [Line Items]
Total investments at fair value		$ 1,669.3	$ 2,149.0	$ 3,118.7

[1] Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at June 30, 2015, $159.7 million was included in "other liabilities," compared to $235.8 million and $31.3 million at June 30, 2014 and December 31, 2014

[2] The total fair value of the portfolio at June 30, 2015 and 2014, and December 31, 2014 included $0.7 billion, $1.1 billion, and $1.9 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.