Investment Securities (Investment Holdings) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment securities
Fixed maturity investments, at fair value	$ 4,931.9	$ 5,196.2
Fixed maturity investments reclassified to assets held for sale		338.1
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	514.6	440.4
Gross unrealized gains	0.4	1.0
Gross unrealized losses	(3.9)	(0.1)
Net foreign currency gains (losses)	1.2	(1.2)
Fixed maturity investments, at fair value	512.3	440.1
Debt securities issued by corporations
Investment securities
Cost or amortized cost	2,295.9	2,321.4
Gross unrealized gains	41.6	88.3
Gross unrealized losses	(20.9)	(1.6)
Net foreign currency gains (losses)	(1.8)	(23.0)
Fixed maturity investments, at fair value	2,314.8	2,385.1
Municipal obligations
Investment securities
Cost or amortized cost	5.3	5.3
Gross unrealized gains	0	0
Gross unrealized losses	(0.3)	(0.1)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	5.0	5.2
Mortgage-backed and asset-backed securities
Investment securities
Cost or amortized cost	1,840.2	2,081.0
Gross unrealized gains	4.3	25.1
Gross unrealized losses	(14.0)	(1.1)
Net foreign currency gains (losses)	4.1	(9.4)
Fixed maturity investments, at fair value	1,834.6	2,095.6
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	447.9	526.6
Gross unrealized gains	4.1	6.9
Gross unrealized losses	(7.7)	(3.0)
Net foreign currency gains (losses)	(4.6)	(8.6)
Fixed maturity investments, at fair value	439.7	521.9
Non-cumulative perpetual preference shares
Investment securities
Cost or amortized cost	79.8	79.9
Gross unrealized gains	6.6	6.7
Gross unrealized losses	0	0
Net foreign currency gains (losses)	(0.1)	(0.2)
Fixed maturity investments, at fair value	86.3	86.4
Common Stock
Investment securities
Cost or amortized cost	852.4	895.2
Gross unrealized gains	189.4	143.4
Gross unrealized losses	(13.2)	(8.8)
Net foreign currency gains (losses)	0.9	(0.1)
Convertible fixed maturity investments
Investment securities
Cost or amortized cost	71.1	121.7
Gross unrealized gains	3.4	6.1
Gross unrealized losses	(0.5)	(0.4)
Net foreign currency gains (losses)	0.1	0
Other long-term investments.
Investment securities
Cost or amortized cost	257.7	257.2
Gross unrealized gains	72.6	65.9
Gross unrealized losses	(25.4)	(22.8)
Net foreign currency gains (losses)	(1.6)	(6.1)
Fixed Income Investments
Investment securities
Cost or amortized cost	5,183.7	5,454.6
Gross unrealized gains	57.0	128.0
Gross unrealized losses	(46.8)	(5.9)
Net foreign currency gains (losses)	(1.2)	(42.4)
Total fixed maturity investments including assets held for sale	5,192.7	5,534.3
OneBeacon Runoff
Investment securities
Fixed maturity investments reclassified to assets held for sale	$ (260.8)	$ (338.1)