Fair Value - Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 51,929.5	$ 51,514.1	$ 50,942.8
Fair Value	6,028.9	5,857.4	6,446.7
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	47,784.3	44,815.7	44,741.6
Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	43,172.5	43,873.1	43,031.0
Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	31,038.8	31,383.5	32,569.1
Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,719.2	18,488.2	19,437.7
Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,135.4	2,185.3	2,440.5
Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16.4	17.9	12.7
Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,167.8	10,692.1	10,678.2
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	11,929.9	12,307.9	10,278.4
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	799.3	790.0	671.3
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,094.6	6,535.6	5,708.1
Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,036.0	4,982.3	3,899.0
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	203.8	181.7	183.5
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	58.7	50.9	51.3
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	8.8	0.0	0.0
Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	136.3	130.8	132.2
Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,611.8	942.6	1,710.6
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,145.2	6,698.4	6,201.2
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,360.5	1,639.9	1,609.8
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,252.4	1,497.4	1,507.3
Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	67.3	82.9	42.5
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40.8	59.6	60.0
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,784.7	5,058.5	4,591.4
Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,766.2	5,041.6	4,579.8
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18.5	16.9	11.6
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	26,083.5	24,548.2	25,754.0
Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23,278.9	19,441.3	21,128.3
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,729.1	18,498.9	19,448.5
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,719.2	18,488.2	19,437.7
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,719.2	18,488.2	19,437.7
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9.9	10.7	10.8
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9.9	10.7	10.8
Fair Value, Inputs, Level 1 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,549.8	942.4	1,679.8
Fair Value, Inputs, Level 1 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,804.6	5,106.9	4,625.7
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	87.8	115.3	98.4
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	87.8	115.3	98.4
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 1 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,716.8	4,991.6	4,527.3
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,716.8	4,991.6	4,527.3
Fair Value, Inputs, Level 1 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25,745.6	26,838.2	25,013.9
Fair Value	6,028.9	5,857.4	6,446.7
Fair Value, Inputs, Level 2 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	24,505.4	25,374.4	23,613.3
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	24,443.4	25,374.2	23,582.5
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	12,319.6	12,895.3	13,131.4
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,135.4	2,185.3	2,440.5
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16.4	17.9	12.7
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,167.8	10,692.1	10,678.2
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	11,929.9	12,307.9	10,278.4
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	799.3	790.0	671.3
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,094.6	6,535.6	5,708.1
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,036.0	4,982.3	3,899.0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	193.9	171.0	172.7
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	58.7	50.9	51.3
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	8.8	0.0	0.0
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	126.4	120.1	121.4
Fair Value, Inputs, Level 2 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	62.0	0.2	30.8
Fair Value, Inputs, Level 2 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,240.2	1,463.8	1,400.6
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,190.8	1,413.8	1,400.6
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,099.7	1,305.7	1,332.5
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	67.3	82.9	42.5
Fair Value, Inputs, Level 2 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	23.8	25.2	25.6
Fair Value, Inputs, Level 2 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	49.4	50.0	0.0
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	49.4	50.0	0.0
Fair Value, Inputs, Level 2 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	100.4	127.7	174.9
Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	100.4	127.7	174.9
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	81.9	110.8	110.8
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	64.9	76.4	76.4
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	0.0
Fair Value, Inputs, Level 3 | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	17.0	34.4	34.4
Fair Value, Inputs, Level 3 | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18.5	16.9	64.1
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0.0	0.0	52.5
Fair Value, Inputs, Level 3 | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18.5	16.9	11.6
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	52,946.7	47,572.7	46,838.2
Long-term Debt	6,386.5	4,898.8	5,397.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	50,640.6	44,736.8	44,088.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	46,028.8	43,794.2	42,378.0
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	33,001.5	31,292.8	32,045.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	19,832.3	18,586.1	19,339.3
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,281.9	2,162.6	2,379.1
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	17.7	17.9	12.7
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Debt Securities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,869.6	10,526.2	10,314.8
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	12,805.3	12,330.5	10,161.0
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	826.5	787.7	666.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,739.4	6,561.0	5,628.2
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Asset-backed Securities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,239.4	4,981.8	3,865.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	222.0	170.9	171.1
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	62.5	50.7	50.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10.5	0.0	0.0
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Total fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	149.0	120.2	120.2
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,611.8	942.6	1,710.6
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,306.1	2,835.9	2,749.6
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,522.5	1,571.8	1,511.2
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,402.5	1,451.7	1,431.2
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	79.9	80.0	39.9
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	40.1	40.1	40.1
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	783.6	1,264.1	1,238.4
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common Stocks, excluding Other Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	765.1	1,247.2	1,226.8
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 18.5	$ 16.9	$ 11.6