Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 45,644.7	$ 40,807.6	$ 39,750.6
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	15.2	15.2	4.6
Total available-for-sale securities	$ 46,134.4	$ 42,029.4	$ 40,981.3
% of Total Fair Value	88.20%	88.40%	89.60%
Equity securities, at Fair Value
Cost	$ 2,718.3	$ 2,546.0	$ 2,432.7
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	3,434.7	2,954.9	2,350.3
Fair Value	$ 6,153.0	$ 5,500.9	$ 4,783.0
% of Total Fair Value	11.80%	11.60%	10.40%
Debt and Equity Securities, at Fair Value
Cost	$ 48,363.0	$ 43,353.6	$ 42,183.3
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	3,449.9	2,970.1	2,354.9
Fair Value	$ 52,287.4	$ 47,530.3	$ 45,764.3
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 399.7	$ 95.5	$ 469.2
Total fair value of non-insurance subsidiary portfolio	2,900.0	6,200.0	2,800.0
Reclassification, Equity Conversion		25.0	25.0
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,479.9	1,333.7	1,277.7
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	92.9	89.2	20.5
Fair Value	$ 1,572.8	$ 1,422.9	$ 1,298.2
% of Total Fair Value	3.00%	3.00%	2.80%
Common equities
Equity securities, at Fair Value
Cost	$ 1,238.4	$ 1,212.3	$ 1,155.0
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	3,341.8	2,865.7	2,329.8
Fair Value	$ 4,580.2	$ 4,078.0	$ 3,484.8
% of Total Fair Value	8.80%	8.60%	7.60%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 44,556.0	$ 35,589.1	$ 35,082.8
Gross Unrealized Gains	612.6	1,223.9	1,266.6
Gross Unrealized Losses	(138.1)	(17.3)	(40.5)
Net Holding Period Gains (Losses)	15.2	15.2	4.6
Total available-for-sale securities	$ 45,045.7	$ 36,810.9	$ 36,313.5
% of Total Fair Value	86.10%	77.40%	79.40%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 21,093.1	$ 12,437.9	$ 9,835.1
Gross Unrealized Gains	144.9	305.8	370.3
Gross Unrealized Losses	(95.9)	(3.7)	(1.6)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 21,142.1	$ 12,740.0	$ 10,203.8
% of Total Fair Value	40.30%	26.80%	22.30%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,123.8	$ 3,099.4	$ 4,382.1
Gross Unrealized Gains	48.8	123.1	148.2
Gross Unrealized Losses	(6.7)	(0.7)	(0.5)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 2,165.9	$ 3,221.8	$ 4,529.8
% of Total Fair Value	4.10%	6.80%	9.90%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 10,568.7	$ 9,579.7	$ 10,038.7
Gross Unrealized Gains	314.6	601.7	574.7
Gross Unrealized Losses	(10.5)	(0.1)	(3.6)
Net Holding Period Gains (Losses)	1.2	3.9	2.2
Total available-for-sale securities	$ 10,874.0	$ 10,185.2	$ 10,612.0
% of Total Fair Value	20.80%	21.40%	23.20%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 624.4	$ 503.3	$ 525.4
Gross Unrealized Gains	3.4	7.1	7.0
Gross Unrealized Losses	(0.9)	(0.9)	(1.4)
Net Holding Period Gains (Losses)	1.0	0.0	0.0
Total available-for-sale securities	$ 627.9	$ 509.5	$ 531.0
% of Total Fair Value	1.20%	1.10%	1.20%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 5,739.4	$ 6,042.6	$ 5,964.4
Gross Unrealized Gains	70.2	142.5	117.1
Gross Unrealized Losses	(20.6)	(10.0)	(27.9)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 5,789.0	$ 6,175.1	$ 6,053.6
% of Total Fair Value	11.10%	13.00%	13.20%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,235.6	$ 3,745.0	$ 4,155.8
Gross Unrealized Gains	29.4	40.1	46.8
Gross Unrealized Losses	(2.8)	(0.5)	(2.3)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 4,262.2	$ 3,784.6	$ 4,200.3
% of Total Fair Value	8.20%	7.90%	9.20%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 171.0	$ 181.2	$ 181.3
Gross Unrealized Gains	1.3	3.6	2.5
Gross Unrealized Losses	(0.7)	(1.4)	(3.2)
Net Holding Period Gains (Losses)	13.0	11.3	2.4
Total available-for-sale securities	$ 184.6	$ 194.7	$ 183.0
% of Total Fair Value	0.40%	0.40%	0.40%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,088.7	$ 5,218.5	$ 4,667.8
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	$ 1,088.7	$ 5,218.5	$ 4,667.8
% of Total Fair Value	2.10%	11.00%	10.20%