Investments - Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 75,351.7	$ 64,231.8	$ 62,285.8
Gross Unrealized Gains	1,072.3	312.2	3.7
Gross Unrealized Losses	(1,257.4)	(2,346.8)	(3,846.3)
Net Holding Period Gains (Losses)	1.1	(29.1)	(56.1)
Total available-for-sale securities	$ 75,167.7	$ 62,168.1	$ 58,387.1
% of Total Fair Value	94.70%	94.20%	94.40%
Equity securities, at Fair Value
Cost	$ 1,493.2	$ 1,683.1	$ 1,639.5
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	2,738.8	2,147.4	1,843.9
Fair Value	$ 4,232.0	$ 3,830.5	$ 3,483.4
% of Total Fair Value	5.30%	5.80%	5.60%
Debt and Equity Securities, at Fair Value
Cost	$ 76,844.9	$ 65,914.9	$ 63,925.3
Gross Unrealized Gains	1,072.3	312.2	3.7
Gross Unrealized Losses	(1,257.4)	(2,346.8)	(3,846.3)
Net Holding Period Gains (Losses)	2,739.9	2,118.3	1,787.8
Fair Value	$ 79,399.7	$ 65,998.6	$ 61,870.5
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 468.5		$ 167.3
Other receivable from broker-dealer and clearing organization		$ 45.6
Total fair value of non-insurance subsidiary portfolio	4,100.0	4,200.0	4,100.0
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	759.7	977.1	976.9
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	(24.7)	(75.0)	(108.0)
Fair Value	$ 735.0	$ 902.1	$ 868.9
% of Total Fair Value	0.90%	1.40%	1.40%
Common equities
Equity securities, at Fair Value
Cost	$ 733.5	$ 706.0	$ 662.6
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	2,763.5	2,222.4	1,951.9
Fair Value	$ 3,497.0	$ 2,928.4	$ 2,614.5
% of Total Fair Value	4.40%	4.40%	4.20%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 74,595.2	$ 62,441.9	$ 60,490.6
Gross Unrealized Gains	1,072.3	312.2	3.7
Gross Unrealized Losses	(1,257.4)	(2,346.8)	(3,846.3)
Net Holding Period Gains (Losses)	1.1	(29.1)	(56.1)
Total available-for-sale securities	$ 74,411.2	$ 60,378.2	$ 56,591.9
% of Total Fair Value	93.70%	91.50%	91.50%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 44,231.3	$ 37,823.2	$ 36,392.2
Gross Unrealized Gains	771.4	204.1	0.0
Gross Unrealized Losses	(574.2)	(1,157.9)	(2,098.4)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 44,428.5	$ 36,869.4	$ 34,293.8
% of Total Fair Value	56.00%	55.90%	55.40%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,681.0	$ 2,338.4	$ 2,257.6
Gross Unrealized Gains	10.7	2.8	0.0
Gross Unrealized Losses	(90.3)	(138.4)	(203.3)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 2,601.4	$ 2,202.8	$ 2,054.3
% of Total Fair Value	3.30%	3.30%	3.30%
Total fixed maturities | Foreign government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 16.9	$ 17.3	$ 16.9
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	(0.5)	(1.0)	(1.5)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 16.4	$ 16.3	$ 15.4
% of Total Fair Value	0.00%	0.10%	0.10%
Total fixed maturities | Corporate and other debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 14,969.8	$ 11,633.7	$ 11,069.8
Gross Unrealized Gains	221.9	87.2	1.3
Gross Unrealized Losses	(155.3)	(334.7)	(596.1)
Net Holding Period Gains (Losses)	(0.9)	(28.8)	(53.7)
Total available-for-sale securities	$ 15,035.5	$ 11,357.4	$ 10,421.3
% of Total Fair Value	18.90%	17.20%	16.90%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,403.5	$ 426.9	$ 530.4
Gross Unrealized Gains	24.8	0.2	0.2
Gross Unrealized Losses	(6.4)	(10.0)	(13.0)
Net Holding Period Gains (Losses)	2.0	0.1	(1.8)
Total available-for-sale securities	$ 1,423.9	$ 417.2	$ 515.8
% of Total Fair Value	1.80%	0.60%	0.80%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,664.4	$ 4,535.2	$ 4,699.5
Gross Unrealized Gains	4.1	2.2	2.1
Gross Unrealized Losses	(376.9)	(597.7)	(723.9)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 4,291.6	$ 3,939.7	$ 3,977.7
% of Total Fair Value	5.40%	6.00%	6.40%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 6,628.3	$ 5,667.2	$ 5,524.2
Gross Unrealized Gains	39.4	15.7	0.1
Gross Unrealized Losses	(53.8)	(107.1)	(210.1)
Net Holding Period Gains (Losses)	0.0	(0.4)	(0.6)
Total available-for-sale securities	$ 6,613.9	$ 5,575.4	$ 5,313.6
% of Total Fair Value	8.30%	8.40%	8.60%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 756.5	$ 1,789.9	$ 1,795.2
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 756.5	$ 1,789.9	$ 1,795.2
% of Total Fair Value	1.00%	2.70%	2.90%