Investment Securities (Investment Holdings Fixed Maturity Table) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,771.9us-gaap_TradingSecuritiesDebt	$ 4,784.3us-gaap_TradingSecuritiesDebt
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	165.3	184.7
Trading Securities, Unrealized Holding Gain	0.3	0.1
Trading Securities, Unrealized Holding Loss	(0.1)	(0.3)
Net foreign currency gains (losses)	0.2	3.6
Fixed maturity investments, at fair value	165.7	188.1
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,103.8	2,221.3
Trading Securities, Unrealized Holding Gain	46.7	45.2
Trading Securities, Unrealized Holding Loss	(1.0)	(5.1)
Net foreign currency gains (losses)	66.2	49.8
Fixed maturity investments, at fair value	2,215.7	2,311.2
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	96.0	82.0
Trading Securities, Unrealized Holding Gain	1.8	1.4
Trading Securities, Unrealized Holding Loss	(0.2)	(0.2)
Net foreign currency gains (losses)	0	0
Fixed maturity investments, at fair value	97.6	83.2
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,049.3	1,811.1
Trading Securities, Unrealized Holding Gain	11.9	7.6
Trading Securities, Unrealized Holding Loss	(2.6)	(3.5)
Net foreign currency gains (losses)	39.5	25.7
Fixed maturity investments, at fair value	2,098.1	1,840.9
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	106.9	274.6
Trading Securities, Unrealized Holding Gain	1.2	4.2
Trading Securities, Unrealized Holding Loss	(0.2)	(1.0)
Net foreign currency gains (losses)	0.9	(2.7)
Fixed maturity investments, at fair value	108.8	275.1
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	79.5	79.6
Trading Securities, Unrealized Holding Gain	6.3	6.1
Trading Securities, Unrealized Holding Loss	0	0
Net foreign currency gains (losses)	0.2	0.1
Fixed maturity investments, at fair value	86.0	85.8
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,600.8	4,653.3
Trading Securities, Unrealized Holding Gain	68.2	64.6
Trading Securities, Unrealized Holding Loss	(4.1)	(10.1)
Net foreign currency gains (losses)	107.0	76.5
Total fixed maturity investments including assets held for sale	$ 4,771.9	$ 4,784.3

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