Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 31,949.1	$ 30,051.8	$ 27,293.0
Gross Unrealized Gains	610.0	115.4	36.8
Gross Unrealized Losses	(11.9)	(249.6)	(304.1)
Net Holding Period Gains (Losses)	1.9	(10.2)	(5.3)
Total available-for-sale securities	$ 32,549.1	$ 29,907.4	$ 27,020.4
% of Total Fair Value	88.40%	89.10%	87.40%
Equity securities, at Fair Value
Cost	$ 2,264.0	$ 2,151.5	$ 1,991.0
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	2,001.5	1,508.5	1,909.8
Fair Value	$ 4,265.5	$ 3,660.0	$ 3,900.8
% of Total Fair Value	11.60%	10.90%	12.60%
Debt and Equity Securities, at Fair Value
Cost	$ 34,213.1	$ 32,203.3	$ 29,284.0
Gross Unrealized Gains	610.0	115.4	36.8
Gross Unrealized Losses	(11.9)	(249.6)	(304.1)
Net Holding Period Gains (Losses)	2,003.4	1,498.3	1,904.5
Fair Value	$ 36,814.6	$ 33,567.4	$ 30,921.2
% of Total Fair Value	100.00%	100.00%	100.00%
Unsettled security transactions, payable	$ 303.5	$ 5.9	$ 362.1
Total fair value of non-insurance subsidiary portfolio	1,200.0	2,900.0	1,700.0
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	1,060.3	1,002.6	677.0
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	69.7	31.3	81.6
Fair Value	$ 1,130.0	$ 1,033.9	$ 758.6
% of Total Fair Value	3.10%	3.10%	2.40%
Common equities
Equity securities, at Fair Value
Cost	$ 1,203.7	$ 1,148.9	$ 1,314.0
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Holding Period Gains (Losses)	1,931.8	1,477.2	1,828.2
Fair Value	$ 3,135.5	$ 2,626.1	$ 3,142.2
% of Total Fair Value	8.50%	7.80%	10.20%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 30,588.2	$ 28,255.9	$ 24,061.8
Gross Unrealized Gains	610.0	115.4	36.8
Gross Unrealized Losses	(11.9)	(249.6)	(304.1)
Net Holding Period Gains (Losses)	1.9	(10.2)	(5.3)
Total available-for-sale securities	$ 31,188.2	$ 28,111.5	$ 23,789.2
% of Total Fair Value	84.70%	83.70%	76.90%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 12,121.9	$ 9,897.4	$ 8,005.8
Gross Unrealized Gains	259.7	71.2	0.9
Gross Unrealized Losses	(2.2)	(52.1)	(141.3)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 12,379.4	$ 9,916.5	$ 7,865.4
% of Total Fair Value	33.60%	29.50%	25.40%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,563.6	$ 1,654.6	$ 1,678.9
Gross Unrealized Gains	27.4	7.3	4.3
Gross Unrealized Losses	(1.3)	(12.8)	(15.9)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 1,589.7	$ 1,649.1	$ 1,667.3
% of Total Fair Value	4.30%	4.90%	5.40%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 7,176.7	$ 8,808.5	$ 7,422.4
Gross Unrealized Gains	210.1	13.6	4.2
Gross Unrealized Losses	(1.8)	(125.3)	(94.3)
Net Holding Period Gains (Losses)	0.7	(2.5)	(2.0)
Total available-for-sale securities	$ 7,385.7	$ 8,694.3	$ 7,330.3
% of Total Fair Value	20.10%	25.90%	23.70%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 660.4	$ 733.5	$ 819.6
Gross Unrealized Gains	6.2	6.0	8.4
Gross Unrealized Losses	(1.4)	(5.1)	(5.4)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 665.2	$ 734.4	$ 822.6
% of Total Fair Value	1.80%	2.20%	2.70%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,361.2	$ 3,332.8	$ 2,725.5
Gross Unrealized Gains	83.3	7.8	2.9
Gross Unrealized Losses	(2.6)	(39.0)	(31.5)
Net Holding Period Gains (Losses)	0.0	0.0	0.0
Total available-for-sale securities	$ 4,441.9	$ 3,301.6	$ 2,696.9
% of Total Fair Value	12.10%	9.80%	8.70%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 4,478.2	$ 3,585.4	$ 3,189.2
Gross Unrealized Gains	19.9	3.6	2.0
Gross Unrealized Losses	(1.0)	(11.8)	(13.7)
Net Holding Period Gains (Losses)	0.1	0.1	0.1
Total available-for-sale securities	$ 4,497.2	$ 3,577.3	$ 3,177.6
% of Total Fair Value	12.20%	10.70%	10.30%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 226.2	$ 243.7	$ 220.4
Gross Unrealized Gains	3.4	5.9	14.1
Gross Unrealized Losses	(1.6)	(3.5)	(2.0)
Net Holding Period Gains (Losses)	1.1	(7.8)	(3.4)
Total available-for-sale securities	$ 229.1	$ 238.3	$ 229.1
% of Total Fair Value	0.60%	0.70%	0.70%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,360.9	$ 1,795.9	$ 3,231.2
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	$ 1,360.9	$ 1,795.9	$ 3,231.2
% of Total Fair Value	3.70%	5.40%	10.50%