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, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 2,343.3	$ 3,009.8
Securities, less than 12 months, gross unrealized losses	34.1	119.9
Securities, 12 months or more, fair value	860.1	1,393.5
Securities, 12 months or more, gross unrealized losses	18.3	25.8
Total, fair value	3,203.4	4,403.3
Total, gross unrealized losses	52.4	145.7
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	448.1	514.4
Securities, less than 12 months, gross unrealized losses	15.0	80.8
Total, fair value	448.1	514.4
Total, gross unrealized losses	15.0	80.8
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	448.1	514.4
Securities, less than 12 months, gross unrealized losses	15.0	80.8
Total, fair value	448.1	514.4
Total, gross unrealized losses	15.0	80.8
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,895.2	2,495.4
Securities, less than 12 months, gross unrealized losses	19.1	39.1
Securities, 12 months or more, fair value	860.1	1,393.5
Securities, 12 months or more, gross unrealized losses	18.3	25.8
Total, fair value	2,755.3	3,888.9
Total, gross unrealized losses	37.4	64.9
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	222.3	270.5
Securities, less than 12 months, gross unrealized losses	1.2	3.1
Securities, 12 months or more, fair value	8.2	16.3
Securities, 12 months or more, gross unrealized losses		0.4
Total, fair value	230.5	286.8
Total, gross unrealized losses	1.2	3.5
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	347.1	105.2
Securities, less than 12 months, gross unrealized losses	3.2	0.8
Securities, 12 months or more, fair value	104.6	372.0
Securities, 12 months or more, gross unrealized losses	5.9	8.3
Total, fair value	451.7	477.2
Total, gross unrealized losses	9.1	9.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	148.5	6.1
Securities, less than 12 months, gross unrealized losses	0.1
Securities, 12 months or more, fair value		5.7
Total, fair value	148.5	11.8
Total, gross unrealized losses	0.1
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	250.8	574.7
Securities, less than 12 months, gross unrealized losses	9.2	17.2
Securities, 12 months or more, fair value	89.9	150.7
Securities, 12 months or more, gross unrealized losses	3.4	4.0
Total, fair value	340.7	725.4
Total, gross unrealized losses	12.6	21.2
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	45.6	133.4
Securities, less than 12 months, gross unrealized losses	1.4	4.1
Securities, 12 months or more, fair value	15.4	26.2
Securities, 12 months or more, gross unrealized losses	0.8	0.4
Total, fair value	61.0	159.6
Total, gross unrealized losses	2.2	4.5
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	245.3	187.9
Securities, less than 12 months, gross unrealized losses	0.8	0.5
Securities, 12 months or more, fair value	423.5	586.4
Securities, 12 months or more, gross unrealized losses	6.0	11.2
Total, fair value	668.8	774.3
Total, gross unrealized losses	6.8	11.7
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	66.0	176.5
Securities, less than 12 months, gross unrealized losses	0.3	0.7
Securities, 12 months or more, fair value	30.3	60.9
Securities, 12 months or more, gross unrealized losses	0.3	0.7
Total, fair value	96.3	237.4
Total, gross unrealized losses	0.6	1.4
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	569.6	1,041.1
Securities, less than 12 months, gross unrealized losses	2.9	12.7
Securities, 12 months or more, fair value	188.2	175.3
Securities, 12 months or more, gross unrealized losses	1.9	0.8
Total, fair value	757.8	1,216.4
Total, gross unrealized losses	$ 4.8	$ 13.5