Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 75,167.7	$ 62,168.1	$ 58,387.1
Equity securities	4,232.0	3,830.5	3,483.4
Investments, fair value	79,399.7	65,998.6	61,870.5
Fair value	6,498.3	6,431.3	5,932.1
Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	735.0	902.1	868.9
Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,497.0	2,928.4	2,614.5
Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,474.5	2,907.8	2,591.9
Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	22.5	20.6	22.6
Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	74,411.2	60,378.2	56,591.9
Total fixed maturities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,428.5	36,869.4	34,293.8
Total fixed maturities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,601.4	2,202.8	2,054.3
Total fixed maturities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	16.4	16.3	15.4
Total fixed maturities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15,035.5	11,357.4	10,421.3
Total fixed maturities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,423.9	417.2	515.8
Total fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	4,291.6	3,939.7	3,977.7
Total fixed maturities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,613.9	5,575.4	5,313.6
Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	62,081.8	50,445.9	46,784.8
Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,428.5	36,869.4	34,293.8
Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,601.4	2,202.8	2,054.3
Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	16.4	16.3	15.4
Total fixed maturities | Debt Securities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15,035.5	11,357.4	10,421.3
Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,329.4	9,932.3	9,807.1
Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,423.9	417.2	515.8
Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	4,291.6	3,939.7	3,977.7
Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,613.9	5,575.4	5,313.6
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	756.5	1,789.9	1,795.2
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	45,183.3	38,626.4	36,057.0
Equity securities	3,452.0	2,885.3	2,573.6
Investments, fair value	48,635.3	41,511.7	38,630.6
Fair value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,452.0	2,885.3	2,573.6
Fair Value, Inputs, Level 1 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	3,452.0	2,885.3	2,573.6
Fair Value, Inputs, Level 1 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,428.5	36,869.4	34,293.8
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,428.5	36,869.4	34,293.8
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,428.5	36,869.4	34,293.8
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Debt Securities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	754.8	1,757.0	1,763.2
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	29,981.4	23,538.7	22,330.1
Equity securities	683.1	838.1	805.2
Investments, fair value	30,664.5	24,376.8	23,135.3
Fair value	6,498.3	6,431.3	5,932.1
Fair Value, Inputs, Level 2 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	683.1	838.1	805.2
Fair Value, Inputs, Level 2 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	29,979.7	23,505.8	22,298.1
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	17,650.3	13,573.5	12,491.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,601.4	2,202.8	2,054.3
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	16.4	16.3	15.4
Fair Value, Inputs, Level 2 | Total fixed maturities | Debt Securities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	15,032.5	11,354.4	10,421.3
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,329.4	9,932.3	9,807.1
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,423.9	417.2	515.8
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	4,291.6	3,939.7	3,977.7
Fair Value, Inputs, Level 2 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,613.9	5,575.4	5,313.6
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1.7	32.9	32.0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3.0	3.0	0.0
Equity securities	96.9	107.1	104.6
Investments, fair value	99.9	110.1	104.6
Fair value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	51.9	64.0	63.7
Fair Value, Inputs, Level 3 | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	45.0	43.1	40.9
Fair Value, Inputs, Level 3 | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	22.5	22.5	18.3
Fair Value, Inputs, Level 3 | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	22.5	20.6	22.6
Fair Value, Inputs, Level 3 | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3.0	3.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3.0	3.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Debt Securities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	3.0	3.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	0.0	0.0	0.0
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	75,351.7	64,231.8	62,285.8
Equity securities	1,493.2	1,683.1	1,639.5
Investments, fair value	76,844.9	65,914.9	63,925.3
Long-term debt	6,891.8	6,888.6	6,887.6
Carrying (Reported) Amount, Fair Value Disclosure | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	759.7	977.1	976.9
Carrying (Reported) Amount, Fair Value Disclosure | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	733.5	706.0	662.6
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	711.0	685.4	640.0
Carrying (Reported) Amount, Fair Value Disclosure | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	22.5	20.6	22.6
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	74,595.2	62,441.9	60,490.6
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	61,899.0	51,812.6	49,736.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	44,231.3	37,823.2	36,392.2
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	2,681.0	2,338.4	2,257.6
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	16.9	17.3	16.9
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Debt Securities | Corporate and other debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	14,969.8	11,633.7	11,069.8
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	12,696.2	10,629.3	10,754.1
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	1,403.5	426.9	530.4
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	4,664.4	4,535.2	4,699.5
Carrying (Reported) Amount, Fair Value Disclosure | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	6,628.3	5,667.2	5,524.2
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total available-for-sale securities	$ 756.5	$ 1,789.9	$ 1,795.2