Reconciliation for Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	$ 102.5	$ 70.9
Transfers into Level 3	7.0	14.5
Transfers out of Level 3	(11.0)	(25.1)
Included in earnings	0.2	(2.1)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	3.3	(1.0)
Purchases	11.3	25.1
Sales	(10.6)	(18.5)
Balance at end of year	102.7	102.5
Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		38.7
Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	75.5	64.6
Transfers into Level 3	6.9	14.5
Transfers out of Level 3	(11.0)	(25.1)
Included in earnings	0.4	(0.7)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	1.5	1.1
Purchases	11.3	25.1
Sales	(9.9)	(18.5)
Balance at end of year	74.7	75.5
Fixed Maturities | Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		14.5
Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	13.6	16.6
Transfers into Level 3	2.6
Included in earnings		(0.1)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	1.1	0.2
Purchases	3.0
Sales	(0.9)	(3.1)
Balance at end of year	19.4	13.6
Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	23.8	28.2
Transfers into Level 3	4.3	14.5
Transfers out of Level 3	(2.3)	(17.3)
Included in earnings	0.4	(0.7)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.7	0.3
Purchases	1.5	11.8
Sales	(2.0)	(13.1)
Balance at end of year	26.4	23.8
Fixed Maturities | Corporate | Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		0.1
Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	5.2	0.8
Transfers out of Level 3		(0.5)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	0.1	(0.1)
Purchases	0.1
Sales	(4.7)	(0.6)
Balance at end of year	0.7	5.2
Fixed Maturities | Residential Mortgage-Backed | Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		5.6
Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	23.7	19.0
Transfers out of Level 3		(7.3)
Included in earnings		0.1
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	(0.4)	1.1
Purchases	5.2	12.3
Sales	(1.8)	(1.5)
Balance at end of year	26.7	23.7
Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	9.2
Transfers out of Level 3	(8.7)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities		(0.4)
Purchases	1.5	1.0
Sales	(0.5)	(0.2)
Balance at end of year	1.5	9.2
Fixed Maturities | Asset-Backed | Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		8.8
Equities and Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at beginning of period	27.0	6.3
Transfers into Level 3	0.1
Included in earnings	(0.2)	(1.4)
Included in other comprehensive income-net appreciation (depreciation) on available-for-sale securities	1.8	(2.1)
Sales	(0.7)
Balance at end of year	28.0	27.0
Equities and Other | Chaucer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition		$ 24.2