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, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	$ 3,909.2	$ 6,963.3
Securities, less than 12 months, gross unrealized losses	88.7	213.3
Securities, 12 months or more, fair value	668.2	455.3
Securities, 12 months or more, gross unrealized losses	38.8	27.8
Total, fair value	4,577.4	7,418.6
Total, gross unrealized losses	127.5	241.1
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	256.4	148.7
Securities, less than 12 months, gross unrealized losses	9.4	1.9
Total, fair value	256.4	148.7
Total, gross unrealized losses	9.4	1.9
Equity Securities | Common Stock
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	256.4	148.7
Securities, less than 12 months, gross unrealized losses	9.4	1.9
Total, fair value	256.4	148.7
Total, gross unrealized losses	9.4	1.9
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	3,652.8	6,814.6
Securities, less than 12 months, gross unrealized losses	79.3	211.4
Securities, 12 months or more, fair value	668.2	455.3
Securities, 12 months or more, gross unrealized losses	38.8	27.8
Total, fair value	4,321.0	7,269.9
Total, gross unrealized losses	118.1	239.2
Debt Securities | U.S. Government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	145.3	698.9
Securities, less than 12 months, gross unrealized losses	2.9	29.6
Securities, 12 months or more, fair value	4.9	4.8
Securities, 12 months or more, gross unrealized losses	0.4	0.5
Total, fair value	150.2	703.7
Total, gross unrealized losses	3.3	30.1
Debt Securities | Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	1,286.4	2,426.2
Securities, less than 12 months, gross unrealized losses	35.9	105.2
Securities, 12 months or more, fair value	253.2	117.8
Securities, 12 months or more, gross unrealized losses	18.2	6.9
Total, fair value	1,539.6	2,544.0
Total, gross unrealized losses	54.1	112.1
Debt Securities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	285.2	625.6
Securities, less than 12 months, gross unrealized losses	2.6	8.2
Securities, 12 months or more, fair value	8.9	8.8
Securities, 12 months or more, gross unrealized losses	0.3	0.3
Total, fair value	294.1	634.4
Total, gross unrealized losses	2.9	8.5
Debt Securities | U.S. corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	560.5	920.0
Securities, less than 12 months, gross unrealized losses	11.3	21.1
Securities, 12 months or more, fair value	36.6	34.2
Securities, 12 months or more, gross unrealized losses	2.0	2.6
Total, fair value	597.1	954.2
Total, gross unrealized losses	13.3	23.7
Debt Securities | Foreign corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	175.3	528.3
Securities, less than 12 months, gross unrealized losses	1.4	5.8
Securities, 12 months or more, fair value	1.5	4.9
Securities, 12 months or more, gross unrealized losses	0.1	0.1
Total, fair value	176.8	533.2
Total, gross unrealized losses	1.5	5.9
Debt Securities | Mortgage and asset backed securities | RMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	538.4	779.4
Securities, less than 12 months, gross unrealized losses	19.4	30.7
Securities, 12 months or more, fair value	329.2	267.9
Securities, 12 months or more, gross unrealized losses	17.5	17.1
Total, fair value	867.6	1,047.3
Total, gross unrealized losses	36.9	47.8
Debt Securities | Mortgage and asset backed securities | CMBS
Schedule of Available-for-sale Securities [Line Items]
Securities, less than 12 months, fair value	207.8	375.2
Securities, less than 12 months, gross unrealized losses	4.1	8.5
Securities, 12 months or more, fair value	26.4	16.9
Securities, 12 months or more, gross unrealized losses	0.3	0.3
Total, fair value	234.2	392.1
Total, gross unrealized losses	4.4	8.8
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	453.9	461.0
Securities, less than 12 months, gross unrealized losses	1.7	2.3
Securities, 12 months or more, fair value	7.5
Total, fair value	461.4	461.0
Total, gross unrealized losses	$ 1.7	$ 2.3