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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 127.0	$ 297.8
Trading Securities, Unrealized Holding Gain	0.0	0.0
Trading Securities, Unrealized Holding Loss	(1.6)	(1.3)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	125.4	296.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	509.4	867.6
Trading Securities, Unrealized Holding Gain	0.2	2.9
Trading Securities, Unrealized Holding Loss	(9.6)	(4.3)
Net foreign currency gains (losses)	0.0	14.7
Fixed maturity investments, at fair value	500.0	880.9
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	281.1	252.0
Trading Securities, Unrealized Holding Gain	0.4	3.7
Trading Securities, Unrealized Holding Loss	(3.5)	(0.8)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	278.0	254.9
Mortgage-backed and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	132.4	697.2
Trading Securities, Unrealized Holding Gain	0.0	1.6
Trading Securities, Unrealized Holding Loss	(3.9)	(4.1)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	128.5	694.7
Foreign government, agency and provincial obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost		2.6
Trading Securities, Unrealized Holding Gain		0.0
Trading Securities, Unrealized Holding Loss		0.0
Net foreign currency gains (losses)		0.1
Fixed maturity investments, at fair value		2.7
Fixed Income Investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	1,049.9	2,117.2
Trading Securities, Unrealized Holding Gain	0.6	8.2
Trading Securities, Unrealized Holding Loss	(18.6)	(10.5)
Net foreign currency gains (losses)	0.0	14.8
Total fixed maturity investments including assets held for sale	$ 1,031.9	$ 2,129.7

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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