Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 40,807.1	$ 40,807.6	$ 36,285.3
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	4.3	15.2	(23.4)
Total available-for-sale securities	$ 41,334.9	$ 42,029.4	$ 36,800.8
% of Total Fair Value	87.20%	88.40%	91.20%
Equity securities, at Fair Value
Cost	$ 2,658.4	$ 2,546.0	$ 2,130.7
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	3,407.3	2,954.9	1,410.8
Fair Value	$ 6,065.7	$ 5,500.9	$ 3,541.5
% of Total Fair Value	12.80%	11.60%	8.80%
Debt and Equity Securities, at Fair Value
Cost	$ 43,465.5	$ 43,353.6	$ 38,416.0
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	3,411.6	2,970.1	1,387.4
Fair Value	$ 47,400.6	$ 47,530.3	$ 40,342.3
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 363.1	$ 95.5	$ 598.0
Total fair value of non-insurance subsidiary portfolio	3,700.0	6,200.0	2,600.0
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,453.9	1,358.7	1,017.5
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	53.3	89.2	(84.1)
Fair Value	$ 1,507.2	$ 1,447.9	$ 933.4
% of Total Fair Value	3.20%	3.10%	2.30%
Common equities
Equity securities, at Fair Value
Cost	$ 1,204.5	$ 1,187.3	$ 1,113.2
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	3,354.0	2,865.7	1,494.9
Fair Value	$ 4,558.5	$ 4,053.0	$ 2,608.1
% of Total Fair Value	9.60%	8.50%	6.50%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 38,564.0	$ 35,589.1	$ 33,761.1
Gross Unrealized Gains	723.1	1,223.9	916.1
Gross Unrealized Losses	(199.6)	(17.3)	(377.2)
Net Holding Period Gains (Losses)	4.3	15.2	(23.4)
Total available-for-sale securities	$ 39,091.8	$ 36,810.9	$ 34,276.6
% of Total Fair Value	82.50%	77.40%	84.90%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 16,002.7	$ 12,437.9	$ 10,841.2
Gross Unrealized Gains	181.7	305.8	654.5
Gross Unrealized Losses	(111.0)	(3.7)	0.0
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 16,073.4	$ 12,740.0	$ 11,495.7
% of Total Fair Value	33.90%	26.80%	28.50%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,574.8	$ 3,099.4	$ 2,243.9
Gross Unrealized Gains	65.3	123.1	64.8
Gross Unrealized Losses	(16.0)	(0.7)	(4.5)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 2,624.1	$ 3,221.8	$ 2,304.2
% of Total Fair Value	5.60%	6.80%	5.70%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,095.2	$ 9,579.7	$ 9,452.9
Gross Unrealized Gains	368.1	601.7	160.8
Gross Unrealized Losses	(24.0)	(0.1)	(120.1)
Net Holding Period Gains (Losses)	(3.1)	3.9	(1.7)
Total available-for-sale securities	$ 10,436.2	$ 10,185.2	$ 9,491.9
% of Total Fair Value	22.00%	21.40%	23.50%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 548.3	$ 503.3	$ 574.3
Gross Unrealized Gains	5.7	7.1	1.5
Gross Unrealized Losses	(0.8)	(0.9)	(19.2)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 553.2	$ 509.5	$ 556.6
% of Total Fair Value	1.20%	1.10%	1.40%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,868.3	$ 6,042.6	$ 5,822.4
Gross Unrealized Gains	68.8	142.5	30.2
Gross Unrealized Losses	(45.1)	(10.0)	(188.3)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 5,892.0	$ 6,175.1	$ 5,664.3
% of Total Fair Value	12.40%	13.00%	14.00%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 3,293.5	$ 3,745.0	$ 4,674.9
Gross Unrealized Gains	30.6	40.1	4.3
Gross Unrealized Losses	(0.7)	(0.5)	(40.0)
Net Holding Period Gains (Losses)	0.0	0.0	(0.1)
Total available-for-sale securities	$ 3,323.4	$ 3,784.6	$ 4,639.1
% of Total Fair Value	7.00%	7.90%	11.50%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 181.2	$ 181.2	$ 151.5
Gross Unrealized Gains	2.9	3.6	0.0
Gross Unrealized Losses	(2.0)	(1.4)	(5.1)
Net Holding Period Gains (Losses)	7.4	11.3	(21.6)
Total available-for-sale securities	$ 189.5	$ 194.7	$ 124.8
% of Total Fair Value	0.40%	0.40%	0.30%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,243.1	$ 5,218.5	$ 2,524.2
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	$ 2,243.1	$ 5,218.5	$ 2,524.2
% of Total Fair Value	4.70%	11.00%	6.30%