Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 24,179.7	$ 22,031.7	$ 20,880.1
Gross Unrealized Gains	1,857.7	1,588.9	1,602.0
Gross Unrealized Losses	(62.1)	(139.8)	(105.7)
Net Realized Gains (Losses)	3.1	1.8	3.4
Fair Value	$ 25,978.4	$ 23,482.6	$ 22,379.8
% of Total Fair Value	100.00%	100.00%	100.00%
Other Payables to Broker-Dealers and Clearing Organizations	$ 287.1	$ 27.8	$ 246.9
Securities Held In Consolidated Non Insurance Subsidiary	1,100.0	1,300.0	600.0
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	18,311.7	16,287.1	13,409.6
Gross Unrealized Gains	122.4	76.5	256.7
Gross Unrealized Losses	(49.1)	(121.6)	(79.0)
Net Realized Gains (Losses)	3.1	1.8	2.2
Fair Value	$ 18,388.1	$ 16,243.8	$ 13,589.5
% of Total Fair Value	70.80%	69.20%	60.70%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,166.4	$ 2,899.2	$ 1,276.0
Gross Unrealized Gains	4.4	0.0	20.0
Gross Unrealized Losses	(15.8)	(29.1)	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 4,155.0	$ 2,870.1	$ 1,296.0
% of Total Fair Value	16.00%	12.20%	5.80%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,473.3	$ 2,509.5	$ 2,545.5
Gross Unrealized Gains	32.5	13.8	72.7
Gross Unrealized Losses	(5.9)	(20.7)	(0.6)
Net Realized Gains (Losses)	0.2	0.0	0.0
Fair Value	$ 2,500.1	$ 2,502.6	$ 2,617.6
% of Total Fair Value	9.70%	10.70%	11.70%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 22.5	$ 24.5	$ 24.9
Gross Unrealized Gains	0.0	0.0	0.1
Gross Unrealized Losses	0.0	0.0	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 22.5	$ 24.5	$ 25.0
% of Total Fair Value	0.10%	0.10%	0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,991.6	$ 4,557.8	$ 3,833.9
Gross Unrealized Gains	29.4	17.3	80.2
Gross Unrealized Losses	(7.6)	(24.3)	(3.7)
Net Realized Gains (Losses)	0.4	0.1	0.6
Fair Value	$ 5,013.8	$ 4,550.9	$ 3,911.0
% of Total Fair Value	19.30%	19.40%	17.50%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,222.8	$ 1,448.5	$ 1,672.8
Gross Unrealized Gains	10.9	23.7	22.7
Gross Unrealized Losses	(5.4)	(15.0)	(21.8)
Net Realized Gains (Losses)	2.3	1.5	1.2
Fair Value	$ 1,230.6	$ 1,458.7	$ 1,674.9
% of Total Fair Value	4.70%	6.20%	7.50%
Fixed maturities | Agency residential pass-through obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 37.5	$ 41.2	$ 46.4
Gross Unrealized Gains	0.0	0.0	0.5
Gross Unrealized Losses	(0.5)	(0.6)	0.0
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 37.0	$ 40.6	$ 46.9
% 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	$ 2,364.9	$ 2,266.9	$ 2,177.3
Gross Unrealized Gains	19.8	12.0	38.8
Gross Unrealized Losses	(9.7)	(25.5)	(5.7)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 2,375.0	$ 2,253.4	$ 2,210.4
% of Total Fair Value	9.10%	9.60%	9.90%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,843.3	$ 2,350.7	$ 1,567.7
Gross Unrealized Gains	6.4	4.6	4.5
Gross Unrealized Losses	(2.5)	(4.4)	(1.0)
Net Realized Gains (Losses)	0.2	0.2	0.4
Fair Value	$ 2,847.4	$ 2,351.1	$ 1,571.6
% of Total Fair Value	11.00%	10.00%	7.00%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 189.4	$ 188.8	$ 265.1
Gross Unrealized Gains	19.0	5.1	17.2
Gross Unrealized Losses	(1.7)	(2.0)	(46.2)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 206.7	$ 191.9	$ 236.1
% of Total Fair Value	0.80%	0.80%	1.00%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 666.0	$ 734.2	$ 745.5
Gross Unrealized Gains	123.6	135.4	130.2
Gross Unrealized Losses	(6.5)	(16.1)	(18.4)
Net Realized Gains (Losses)	0.0	0.0	1.2
Fair Value	$ 783.1	$ 853.5	$ 858.5
% of Total Fair Value	3.00%	3.60%	3.80%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,472.3	$ 1,437.5	$ 1,558.6
Gross Unrealized Gains	1,611.7	1,377.0	1,215.1
Gross Unrealized Losses	(6.5)	(2.1)	(8.3)
Net Realized Gains (Losses)	0.0	0.0	0.0
Fair Value	$ 3,077.5	$ 2,812.4	$ 2,765.4
% of Total Fair Value	11.80%	12.00%	12.40%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 3,729.7	$ 3,572.9	$ 5,166.4
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	$ 3,729.7	$ 3,572.9	$ 5,166.4
% of Total Fair Value	14.40%	15.20%	23.10%