Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fixed maturities:
Fair Value	$ 27,020.4	$ 23,071.1	$ 22,117.8
Total available-for-sale securities
Cost	27,293.0
Gross Unrealized Gains	36.8
Gross Unrealized Losses	304.1
Net Holding Period Gains (Losses)	(5.3)
Fair Value	$ 27,020.4
% of Total Fair Value	87.40%
Equity securities:
Cost	$ 1,991.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Net Holding Period Gains (Losses)	1,909.8
Fair Value	$ 3,900.8	4,203.6	3,860.6
% of Total Fair Value	12.60%
Total portfolio
Cost	$ 29,284.0
Gross Unrealized Gains	36.8
Gross Unrealized Losses	(304.1)
Net Holding Period Gains (Losses)	1,904.5
Fair Value	$ 30,921.2	27,274.7	25,978.4
% of Total Fair Value	100.00%
Available for Sale Securities
Cost		25,276.9	24,179.7
Gross Unrealized Gains		2,097.0	1,857.7
Gross Unrealized Losses		99.4	62.1
Net Holding Period Gains (Losses)		0.2	3.1
Fair Value		$ 27,274.7	$ 25,978.4
% of Total Fair Value		100.00%	100.00%
Unsettled security transactions	$ 362.1		$ 287.1
Unsettled security transactions		$ 5.8
Total fair value of portfolio	1,700.0	1,600.0	1,100.0
Nonredeemable preferred stocks
Equity securities:
Cost	677.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Net Holding Period Gains (Losses)	81.6
Fair Value	$ 758.6
% of Total Fair Value	2.40%
Available for Sale Securities
Cost		698.6	666.0
Gross Unrealized Gains		114.0	123.6
Gross Unrealized Losses		8.8	6.5
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 803.8	$ 783.1
% of Total Fair Value		2.90%	3.00%
Common equities
Equity securities:
Cost	$ 1,314.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Net Holding Period Gains (Losses)	1,828.2
Fair Value	$ 3,142.2
% of Total Fair Value	10.20%
Available for Sale Securities
Cost		$ 1,499.0	$ 1,472.3
Gross Unrealized Gains		1,901.0	1,611.7
Gross Unrealized Losses		0.2	6.5
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 3,399.8	$ 3,077.5
% of Total Fair Value		12.50%	11.80%
Total fixed maturities
Fixed maturities:
Cost	$ 24,061.8
Gross Unrealized Gains	36.8
Gross Unrealized Losses	(304.1)
Net Holding Period Gains (Losses)	(5.3)
Fair Value	$ 23,789.2
% of Total Fair Value	76.90%
Available for Sale Securities
Cost		$ 20,209.9	$ 18,311.7
Gross Unrealized Gains		82.0	122.4
Gross Unrealized Losses		90.4	49.1
Net Holding Period Gains (Losses)		0.2	3.1
Fair Value		$ 20,201.7	$ 18,388.1
% of Total Fair Value		74.10%	70.80%
Total fixed maturities | U.S. government obligations
Fixed maturities:
Cost	$ 8,005.8
Gross Unrealized Gains	0.9
Gross Unrealized Losses	(141.3)
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 7,865.4
% of Total Fair Value	25.40%
Available for Sale Securities
Cost		$ 6,688.8	$ 4,166.4
Gross Unrealized Gains		1.1	4.4
Gross Unrealized Losses		44.0	15.8
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 6,645.9	$ 4,155.0
% of Total Fair Value		24.40%	16.00%
Total fixed maturities | State and local government obligations
Fixed maturities:
Cost	$ 1,678.9
Gross Unrealized Gains	4.3
Gross Unrealized Losses	(15.9)
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 1,667.3
% of Total Fair Value	5.40%
Available for Sale Securities
Cost		$ 2,285.6	$ 2,473.3
Gross Unrealized Gains		20.7	32.5
Gross Unrealized Losses		9.3	5.9
Net Holding Period Gains (Losses)		0.1	0.2
Fair Value		$ 2,297.1	$ 2,500.1
% of Total Fair Value		8.40%	9.70%
Total fixed maturities | Foreign government obligations
Fixed maturities:
Cost	$ 0.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 0.0
% of Total Fair Value	0.00%
Available for Sale Securities
Cost		$ 0.0	$ 22.5
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 0.0	$ 22.5
% of Total Fair Value		0.00%	0.10%
Total fixed maturities | Corporate debt securities
Fixed maturities:
Cost	$ 7,422.4
Gross Unrealized Gains	4.2
Gross Unrealized Losses	(94.3)
Net Holding Period Gains (Losses)	(2.0)
Fair Value	$ 7,330.3
% of Total Fair Value	23.70%
Available for Sale Securities
Cost		$ 4,997.2	$ 4,991.6
Gross Unrealized Gains		14.8	29.4
Gross Unrealized Losses		14.4	7.6
Net Holding Period Gains (Losses)		0.1	0.4
Fair Value		$ 4,997.7	$ 5,013.8
% of Total Fair Value		18.30%	19.30%
Total fixed maturities | Residential mortgage-backed securities
Fixed maturities:
Cost	$ 819.6
Gross Unrealized Gains	8.4
Gross Unrealized Losses	(5.4)
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 822.6
% of Total Fair Value	2.70%
Available for Sale Securities
Cost		$ 828.8	$ 1,260.3
Gross Unrealized Gains		11.3	10.9
Gross Unrealized Losses		3.4	5.9
Net Holding Period Gains (Losses)		0.0	2.3
Fair Value		$ 836.7	$ 1,267.6
% of Total Fair Value		3.10%	4.80%
Total fixed maturities | Commercial mortgage-backed securities
Fixed maturities:
Cost	$ 2,725.5
Gross Unrealized Gains	2.9
Gross Unrealized Losses	(31.5)
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 2,696.9
% of Total Fair Value	8.70%
Available for Sale Securities
Cost		$ 2,760.1	$ 2,364.9
Gross Unrealized Gains		11.8	19.8
Gross Unrealized Losses		13.3	9.7
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 2,758.6	$ 2,375.0
% of Total Fair Value		10.10%	9.10%
Total fixed maturities | Other asset-backed securities
Fixed maturities:
Cost	$ 3,189.2
Gross Unrealized Gains	2.0
Gross Unrealized Losses	(13.7)
Net Holding Period Gains (Losses)	0.1
Fair Value	$ 3,177.6
% of Total Fair Value	10.30%
Available for Sale Securities
Cost		$ 2,454.5	$ 2,843.3
Gross Unrealized Gains		4.5	6.4
Gross Unrealized Losses		4.5	2.5
Net Holding Period Gains (Losses)		0.2	0.2
Fair Value		$ 2,454.7	$ 2,847.4
% of Total Fair Value		9.00%	11.00%
Total fixed maturities | Redeemable preferred stocks
Fixed maturities:
Cost	$ 220.4
Gross Unrealized Gains	14.1
Gross Unrealized Losses	(2.0)
Net Holding Period Gains (Losses)	(3.4)
Fair Value	$ 229.1
% of Total Fair Value	0.70%
Available for Sale Securities
Cost		$ 194.9	$ 189.4
Gross Unrealized Gains		17.8	19.0
Gross Unrealized Losses		1.5	1.7
Net Holding Period Gains (Losses)		(0.2)	0.0
Fair Value		$ 211.0	$ 206.7
% of Total Fair Value		0.80%	0.80%
Short-term investments
Short-term investments
Cost	$ 3,231.2
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Net Holding Period Gains (Losses)	0.0
Fair Value	$ 3,231.2
% of Total Fair Value	10.50%
Available for Sale Securities
Cost		$ 2,869.4	$ 3,729.7
Gross Unrealized Gains		0.0	0.0
Gross Unrealized Losses		0.0	0.0
Net Holding Period Gains (Losses)		0.0	0.0
Fair Value		$ 2,869.4	$ 3,729.7
% of Total Fair Value		10.50%	14.40%