Investment Securities (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	Dec. 31, 2011
Investment income
Gross investment income	$ 14.4	$ 17.8	$ 46.5	$ 61.1
Less investment expenses	(1.6)	(1.7)	(5.0)	(5.3)
Net investment income	12.8	16.1	41.5	55.8
Net realized investment gains (losses), pre-tax
Net realized investment gains, pre-tax	13.7	22.3	30.1	57.2
Total net changes in fair value reflected in revenues
Changes in net unrealized gains and losses	26.1	(69.3)	27.6	(70.2)
Changes in net foreign currency translation gains and losses	0.2	(0.4)	0.2	(0.3)
Total net changes in fair value reflected in revenues	26.3	(69.7)	27.8	(70.5)
Change in Deferred Gains and Losses	0	(0.1)	(0.2)	(1.5)
Gross unrealized gains and losses
Goss unrealized investment gains	137.9		137.9		123.5
Gross unrealized investment losses	(8.1)		(8.1)		(21.6)
Net unrealized gains from trading investment portfolio, pre-tax	129.8		129.8		101.9
Income taxes on net unrealized investment gains	(43.2)		(43.2)		(33.9)
Total net unrealized investment gains, after tax	86.6		86.6		68.0
Fixed maturity investments
Investment income
Gross investment income	12.0	15.4	38.6	54.8
Net realized investment gains (losses), pre-tax
Net realized investment gains, pre-tax	10.6	6.7	26.7	25.3
Total net changes in fair value reflected in revenues
Changes in net unrealized gains and losses	5.9	(14.7)	8.6	(15.1)
Changes in net foreign currency translation gains and losses	0.2	(0.3)	0.2	(0.2)
Total net changes in fair value reflected in revenues	6.1	(15.0)	8.8	(15.3)
Short-term investments
Investment income
Gross investment income			0.1	0.1
Total net changes in fair value reflected in revenues
Changes in net foreign currency translation gains and losses		(0.1)
Total net changes in fair value reflected in revenues		(0.1)
Common Equity securities
Investment income
Gross investment income	1.5	1.5	4.8	3.9
Net realized investment gains (losses), pre-tax
Net realized investment gains, pre-tax	2.9	14.5	(0.2)	18.7
Total net changes in fair value reflected in revenues
Changes in net unrealized gains and losses	18.7	(47.0)	17.0	(44.1)
Changes in net foreign currency translation gains and losses				(0.1)
Total net changes in fair value reflected in revenues	18.7	(47.0)	17.0	(44.2)
Convertible fixed maturity investments
Investment income
Gross investment income	1.1	0.8	3.4	2.6
Net realized investment gains (losses), pre-tax
Net realized investment gains, pre-tax	0.6	0.3	0.2	4.8
Total net changes in fair value reflected in revenues
Changes in net unrealized gains and losses	(0.6)	(7.1)	(2.0)	(13.4)
Total net changes in fair value reflected in revenues	(0.6)	(7.1)	(2.0)	(13.4)
Other investments
Investment income
Gross investment income	(0.2)	0.1	(0.4)	(0.3)
Net realized investment gains (losses), pre-tax
Net realized investment gains, pre-tax	(0.4)	0.8	1.6	8.4
Total net changes in fair value reflected in revenues
Changes in net unrealized gains and losses	2.1	(0.5)	4.0	2.4
Total net changes in fair value reflected in revenues	$ 2.1	$ (0.5)	$ 4.0	$ 2.4