Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 25,080.7	$ 22,031.7	$ 21,886.9
Gross Unrealized Gains	1,972.6	1,588.9	1,675.4
Gross Unrealized Losses	(60.7)	(139.8)	(45.1)
Net Realized Gains (Losses)	2.1	1.8	4.0
Fair Value	$ 26,994.7	$ 23,482.6	$ 23,521.2
% of Total Fair Value	100.00%	100.00%	100.00%
Other Payables to Broker-Dealers and Clearing Organizations	$ 238.3	$ 27.8	$ 185.3
Securities Held In Consolidated Non Insurance Subsidiary	1,100.0	1,300.0	1,000.0
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	18,583.1	16,287.1	13,708.0
Gross Unrealized Gains	122.7	76.5	232.9
Gross Unrealized Losses	(47.9)	(121.6)	(26.8)
Net Realized Gains (Losses)	2.1	1.8	2.8
Fair Value	$ 18,660.0	$ 16,243.8	$ 13,916.9
% of Total Fair Value	69.10%	69.20%	59.20%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,612.2	$ 2,899.2	$ 726.0
Gross Unrealized Gains	3.0	0.0	8.9
Gross Unrealized Losses	(20.2)	(29.1)	(0.3)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 4,595.0	$ 2,870.1	$ 734.6
% of Total Fair Value	17.00%	12.20%	3.10%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,332.2	$ 2,509.5	$ 2,508.9
Gross Unrealized Gains	35.0	13.8	58.7
Gross Unrealized Losses	(3.1)	(20.7)	(1.6)
Net Realized Gains (Losses)	0.1	0.0	0.0
Fair Value	$ 2,364.2	$ 2,502.6	$ 2,566.0
% of Total Fair Value	8.70%	10.70%	10.90%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 24.2	$ 24.5	$ 25.4
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 24.2	$ 24.5	$ 25.4
% of Total Fair Value	0.10%	0.10%	0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 5,195.7	$ 4,557.8	$ 4,327.5
Gross Unrealized Gains	32.7	17.3	76.9
Gross Unrealized Losses	(4.2)	(24.3)	(1.8)
Net Realized Gains (Losses)	1.4	0.1	0.7
Fair Value	$ 5,225.6	$ 4,550.9	$ 4,403.3
% of Total Fair Value	19.40%	19.40%	18.70%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 911.3	$ 1,448.5	$ 1,582.6
Gross Unrealized Gains	12.2	23.7	25.3
Gross Unrealized Losses	(3.2)	(15.0)	(15.5)
Net Realized Gains (Losses)	0.0	1.5	1.8
Fair Value	$ 920.3	$ 1,458.7	$ 1,594.2
% of Total Fair Value	3.40%	6.20%	6.80%
Fixed maturities | Agency residential pass-through obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 35.7	$ 41.2	$ 43.6
Gross Unrealized Gains	0.0	0.0	0.5
Gross Unrealized Losses	(0.4)	(0.6)	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 35.3	$ 40.6	$ 44.1
% of Total Fair Value	0.10%	0.20%	0.20%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,763.7	$ 2,266.9	$ 2,249.4
Gross Unrealized Gains	17.0	12.0	35.1
Gross Unrealized Losses	(12.8)	(25.5)	(6.1)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 2,767.9	$ 2,253.4	$ 2,278.4
% of Total Fair Value	10.30%	9.60%	9.70%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,485.6	$ 2,350.7	$ 2,037.5
Gross Unrealized Gains	6.4	4.6	7.9
Gross Unrealized Losses	(2.0)	(4.4)	(0.3)
Net Realized Gains (Losses)	0.2	0.2	0.3
Fair Value	$ 2,490.2	$ 2,351.1	$ 2,045.4
% of Total Fair Value	9.20%	10.00%	8.70%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 222.5	$ 188.8	$ 207.1
Gross Unrealized Gains	16.4	5.1	19.6
Gross Unrealized Losses	(2.0)	(2.0)	(1.2)
Net Realized Gains (Losses)	0.4	0.0	0.0
Fair Value	$ 237.3	$ 191.9	$ 225.5
% of Total Fair Value	0.90%	0.80%	1.00%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 700.6	$ 734.2	$ 726.6
Gross Unrealized Gains	120.4	135.4	146.2
Gross Unrealized Losses	(7.3)	(16.1)	(13.9)
Net Realized Gains (Losses)	0.0	0.0	1.2
Fair Value	$ 813.7	$ 853.5	$ 860.1
% of Total Fair Value	3.00%	3.60%	3.60%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,485.5	$ 1,437.5	$ 1,576.1
Gross Unrealized Gains	1,729.5	1,377.0	1,296.3
Gross Unrealized Losses	(5.5)	(2.1)	(4.4)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 3,209.5	$ 2,812.4	$ 2,868.0
% of Total Fair Value	11.90%	12.00%	12.20%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,311.5	$ 3,572.9	$ 5,876.2
Gross Unrealized Gains	0.0	0.0	0.0
Gross Unrealized Losses	0.0	0.0	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 4,311.5	$ 3,572.9	$ 5,876.2
% of Total Fair Value	16.00%	15.20%	25.00%