Investments - Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 67,276.0	$ 64,231.8	$ 55,648.0
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893.0)
Net Holding Period Gains (Losses)	(21.9)	(29.1)	(60.6)
Total available-for-sale securities	$ 64,956.4	$ 62,168.1	$ 52,813.3
% of Total Fair Value	94.10%	94.20%	93.20%
Equity securities, at Fair Value
Cost	$ 1,639.3	$ 1,683.1	$ 1,938.2
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	2,442.3	2,147.4	1,934.9
Fair Value	$ 4,081.6	$ 3,830.5	$ 3,873.1
% of Total Fair Value	5.90%	5.80%	6.80%
Debt and Equity Securities, at Fair Value
Cost	$ 68,915.3	$ 65,914.9	$ 57,586.2
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893.0)
Net Holding Period Gains (Losses)	2,420.4	2,118.3	1,874.3
Fair Value	$ 69,038.0	$ 65,998.6	$ 56,686.4
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 16.2		$ 22.8
Other receivable from broker-dealer and clearing organization		$ 45.6
Total fair value of non-insurance subsidiary portfolio	3,200.0	4,200.0	4,100.0
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	931.1	977.1	1,197.7
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	(44.4)	(75.0)	(118.9)
Fair Value	$ 886.7	$ 902.1	$ 1,078.8
% of Total Fair Value	1.30%	1.40%	1.90%
Common equities
Equity securities, at Fair Value
Cost	$ 708.2	$ 706.0	$ 740.5
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	2,486.7	2,222.4	2,053.8
Fair Value	$ 3,194.9	$ 2,928.4	$ 2,794.3
% of Total Fair Value	4.60%	4.40%	4.90%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 65,949.3	$ 62,441.9	$ 53,123.9
Gross Unrealized Gains	125.5	312.2	118.9
Gross Unrealized Losses	(2,423.2)	(2,346.8)	(2,893.0)
Net Holding Period Gains (Losses)	(21.9)	(29.1)	(60.6)
Total available-for-sale securities	$ 63,629.7	$ 60,378.2	$ 50,289.2
% of Total Fair Value	92.20%	91.50%	88.70%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 39,823.8	$ 37,823.2	$ 28,490.9
Gross Unrealized Gains	66.2	204.1	68.8
Gross Unrealized Losses	(1,327.3)	(1,157.9)	(1,209.6)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 38,562.7	$ 36,869.4	$ 27,350.1
% of Total Fair Value	55.80%	55.90%	48.30%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,200.7	$ 2,338.4	$ 2,222.4
Gross Unrealized Gains	1.5	2.8	0.9
Gross Unrealized Losses	(142.4)	(138.4)	(161.7)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 2,059.8	$ 2,202.8	$ 2,061.6
% of Total Fair Value	3.00%	3.30%	3.60%
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	(1.0)	(1.0)	(1.1)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 15.9	$ 16.3	$ 15.8
% of Total Fair Value	0.10%	0.10%	0.10%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 12,821.9	$ 11,446.0	$ 11,195.8
Gross Unrealized Gains	45.9	87.2	45.6
Gross Unrealized Losses	(328.1)	(332.3)	(525.4)
Net Holding Period Gains (Losses)	(18.4)	(17.2)	(34.7)
Total available-for-sale securities	$ 12,521.3	$ 11,183.7	$ 10,681.3
% of Total Fair Value	18.10%	16.90%	18.80%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 390.1	$ 426.9	$ 655.0
Gross Unrealized Gains	0.2	0.2	0.2
Gross Unrealized Losses	(9.2)	(10.0)	(16.3)
Net Holding Period Gains (Losses)	2.0	0.1	(8.9)
Total available-for-sale securities	$ 383.1	$ 417.2	$ 630.0
% of Total Fair Value	0.60%	0.60%	1.10%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,368.6	$ 4,535.2	$ 5,252.6
Gross Unrealized Gains	2.5	2.2	1.6
Gross Unrealized Losses	(519.3)	(597.7)	(751.2)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 3,851.8	$ 3,939.7	$ 4,503.0
% of Total Fair Value	5.60%	6.00%	7.90%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 6,146.1	$ 5,667.2	$ 5,087.8
Gross Unrealized Gains	9.2	15.7	1.8
Gross Unrealized Losses	(93.9)	(107.1)	(222.5)
Net Holding Period Gains (Losses)	(0.2)	(0.4)	(1.3)
Total available-for-sale securities	$ 6,061.2	$ 5,575.4	$ 4,865.8
% of Total Fair Value	8.80%	8.40%	8.60%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 181.2	$ 187.7	$ 202.5
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	(2.0)	(2.4)	(5.2)
Net Holding Period Gains (Losses)	(5.3)	(11.6)	(15.7)
Total available-for-sale securities	$ 173.9	$ 173.7	$ 181.6
% of Total Fair Value	0.20%	0.30%	0.30%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,326.7	$ 1,789.9	$ 2,524.1
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	$ 1,326.7	$ 1,789.9	$ 2,524.1
% of Total Fair Value	1.90%	2.70%	4.50%