Gross Unrealized Losses and Related Fair Values for Equity Securities and Debt Securities Grouped by Duration of Time in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,250.7	$ 6,557.4
Securities, less than 12 months, gross unrealized losses	117.0	197.5
Securities, 12 months or more, fair value	1,036.8	729.4
Securities, 12 months or more, gross unrealized losses	33.5	35.3
Total, fair value	4,287.5	7,286.8
Total, gross unrealized losses	150.5	232.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,030.5	1,355.6
Securities, less than 12 months, gross unrealized losses	56.4	87.0
Total, fair value	1,030.5	1,355.6
Total, gross unrealized losses	56.4	87.0
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,030.5	1,355.6
Securities, less than 12 months, gross unrealized losses	56.4	87.0
Total, fair value	1,030.5	1,355.6
Total, gross unrealized losses	56.4	87.0
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	2,220.2	5,201.8
Securities, less than 12 months, gross unrealized losses	60.6	110.5
Securities, 12 months or more, fair value	1,036.8	729.4
Securities, 12 months or more, gross unrealized losses	33.5	35.3
Total, fair value	3,257.0	5,931.2
Total, gross unrealized losses	94.1	145.8
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	110.0	818.4
Securities, less than 12 months, gross unrealized losses	0.7	13.9
Securities, 12 months or more, fair value	4.3	7.9
Securities, 12 months or more, gross unrealized losses		0.1
Total, fair value	114.3	826.3
Total, gross unrealized losses	0.7	14.0
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	203.0	276.2
Securities, less than 12 months, gross unrealized losses	1.0	2.4
Securities, 12 months or more, fair value	155.4	108.3
Securities, 12 months or more, gross unrealized losses	7.4	6.4
Total, fair value	358.4	384.5
Total, gross unrealized losses	8.4	8.8
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	152.9	208.5
Securities, less than 12 months, gross unrealized losses	0.2	1.3
Securities, 12 months or more, fair value	2.6
Total, fair value	155.5	208.5
Total, gross unrealized losses	0.2	1.3
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	300.6	1,149.8
Securities, less than 12 months, gross unrealized losses	15.0	39.0
Securities, 12 months or more, fair value	157.3	70.0
Securities, 12 months or more, gross unrealized losses	3.5	9.0
Total, fair value	457.9	1,219.8
Total, gross unrealized losses	18.5	48.0
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	122.2	479.9
Securities, less than 12 months, gross unrealized losses	1.4	10.8
Securities, 12 months or more, fair value	72.7	12.5
Securities, 12 months or more, gross unrealized losses	2.3	1.9
Total, fair value	194.9	492.4
Total, gross unrealized losses	3.7	12.7
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	7.9	511.1
Securities, less than 12 months, gross unrealized losses	0.1	6.5
Securities, 12 months or more, fair value	334.1	250.6
Securities, 12 months or more, gross unrealized losses	1.9	5.9
Total, fair value	342.0	761.7
Total, gross unrealized losses	2.0	12.4
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	372.8	593.1
Securities, less than 12 months, gross unrealized losses	6.5	9.4
Securities, 12 months or more, fair value	35.1	15.1
Securities, 12 months or more, gross unrealized losses	1.7	0.2
Total, fair value	407.9	608.2
Total, gross unrealized losses	8.2	9.6
Debt Securities | Mortgage and asset backed securities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	950.8	1,164.8
Securities, less than 12 months, gross unrealized losses	35.7	27.2
Securities, 12 months or more, fair value	275.3	265.0
Securities, 12 months or more, gross unrealized losses	16.7	11.8
Total, fair value	1,226.1	1,429.8
Total, gross unrealized losses	$ 52.4	$ 39.0