Segment Information (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011	Dec. 31, 2010
Segment Information
Earned premiums	$ 293.9	$ 259.1	$ 846.0	$ 748.0
Loss and LAE	(164.7)	(149.7)	(452.5)	(421.3)
Policy acquisition expenses	(66.6)	(58.6)	(185.6)	(161.5)
Other underwriting expenses	(47.4)	(36.0)	(146.2)	(124.5)
Total underwriting income (loss)	15.2	14.8	61.7	40.7
Net investment income	12.8	16.1	41.5	55.8
Net realized and unrealized investment gains (losses)	40.0	(47.4)	57.9	(13.3)
Net other (expenses) revenues	(0.4)		(0.1)	(12.2)
General and administrative expenses	(4.4)	(2.5)	(9.7)	(7.4)
Interest expense on debt	(4.0)	(4.1)	(12.1)	(16.4)
Pre-tax income (loss) from continuing operations	59.2	(23.1)	139.2	47.2
Segment Information, Additional Disclosure
Total investments	2,273.0		2,273.0			2,707.6
Reinsurance recoverable on paid and unpaid losses	43.3		43.3			2,184.0
Deferred acquisition costs	132.5		132.5			123.5
Ceded unearned premiums	11.3		11.3			10.7
Loss and loss adjustment expense reserves	911.6	3,159.1	911.6	3,159.1	3,175.3	3,358.6	3,131.5	3,295.5
Unearned premiums	610.7		610.7			528.0
Debt	269.8		269.8			269.7
Net written premiums	335.2	297.2	930.4	821.4
Loss and LAE	56.00%	57.80%	56.00%	57.80%
Expense	38.80%	36.50%	38.80%	36.50%
Total GAAP combined	94.80%	94.30%	94.80%	94.30%
Specialty Insurance Operations
Segment Information
Major underwriting unit			3
Earned premiums	293.9	259.1	846.0	747.9
Loss and LAE	(164.7)	(149.7)	(452.5)	(421.3)
Policy acquisition expenses	(66.6)	(58.6)	(185.6)	(161.3)
Other underwriting expenses	(47.4)	(36.0)	(146.2)	(124.4)
Total underwriting income (loss)	15.2	14.8	61.7	40.9
Net other (expenses) revenues	(1.1)	0.3	(0.9)	0.5
General and administrative expenses	(0.6)	(0.4)	(1.4)	(1.3)
Pre-tax income (loss) from continuing operations	13.5	14.7	59.4	40.1
Segment Information, Additional Disclosure
Reinsurance recoverable on paid and unpaid losses	43.1		43.1			62.7
Deferred acquisition costs	132.5		132.5			123.3
Ceded unearned premiums	11.3		11.3			10.5
Loss and loss adjustment expense reserves	911.4		911.4			868.6
Unearned premiums	608.6		608.6			525.6
Net written premiums	335.2	297.2	930.4	821.5
Loss and LAE	56.00%	56.30%	56.00%	56.30%
Expense	38.80%	38.20%	38.80%	38.20%
Total GAAP combined	94.80%	94.50%	94.80%	94.50%
Specialty Insurance Operations | MGA Business
Segment Information
Earned premiums	78.4	69.8	227.5	204.1
Segment Information, Additional Disclosure
Net written premiums	128.7	110.5	263.4	230.7
Loss and LAE	54.20%	50.80%	54.20%	50.80%
Expense	46.30%	42.70%	46.30%	42.70%
Total GAAP combined	100.50%	93.50%	100.50%	93.50%
Specialty Insurance Operations | Specialty Industries
Segment Information
Earned premiums	106.7	90.2	309.0	254.8
Segment Information, Additional Disclosure
Net written premiums	106.6	89.7	337.8	278.2
Loss and LAE	55.40%	54.20%	55.40%	54.20%
Expense	36.60%	39.20%	36.60%	39.20%
Total GAAP combined	92.00%	93.40%	92.00%	93.40%
Specialty Insurance Operations | Specialty Products
Segment Information
Earned premiums	108.8	99.1	309.5	289.0
Segment Information, Additional Disclosure
Net written premiums	99.9	97.0	329.2	312.6
Loss and LAE	57.90%	62.20%	57.90%	62.20%
Expense	35.70%	34.10%	35.70%	34.10%
Total GAAP combined	93.60%	96.30%	93.60%	96.30%
Other Insurance Operations
Segment Information
Earned premiums				0.1
Policy acquisition expenses				(0.2)
Other underwriting expenses				(0.1)
Total underwriting income (loss)				(0.2)
Net other (expenses) revenues	0.1		0.3
Pre-tax income (loss) from continuing operations	0.1		0.3	(0.2)
Segment Information, Additional Disclosure
Reinsurance recoverable on paid and unpaid losses	0.2		0.2			2,121.3
Deferred acquisition costs						0.2
Ceded unearned premiums						0.2
Loss and loss adjustment expense reserves	0.2		0.2			2,490.0
Unearned premiums	2.1		2.1			2.4
Net written premiums				(0.1)
Investing, Financing and Corporate Operations
Segment Information
Net investment income	12.8	16.1	41.5	55.8
Net realized and unrealized investment gains (losses)	40.0	(47.4)	57.9	(13.3)
Net other (expenses) revenues	(0.6)	(0.3)	(0.5)	(12.7)
General and administrative expenses	(3.8)	(2.1)	(8.3)	(6.1)
Interest expense on debt	(4.0)	(4.1)	(12.1)	(16.4)
Pre-tax income (loss) from continuing operations	44.4	(37.8)	78.5	7.3
Segment Information, Additional Disclosure
Total investments	227.3		227.3			2,707.6
Debt	$ 269.8		$ 269.8			$ 269.7