Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,490.0	$ 2,081.1
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	218.1	112.1
Trading Securities, Unrealized Holding Gain	0.1	0.0
Trading Securities, Unrealized Holding Loss	(0.4)	(1.1)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	217.8	111.0
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	608.3	752.0
Trading Securities, Unrealized Holding Gain	3.2	2.3
Trading Securities, Unrealized Holding Loss	(1.0)	(10.1)
Net foreign currency gains (losses)	14.5	2.1
Fixed maturity investments, at fair value	625.0	746.3
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	252.1	238.7
Trading Securities, Unrealized Holding Gain	3.1	1.1
Trading Securities, Unrealized Holding Loss	(0.5)	(1.3)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	254.7	238.5
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	389.1	986.9
Trading Securities, Unrealized Holding Gain	1.2	0.8
Trading Securities, Unrealized Holding Loss	(2.4)	(7.9)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	387.9	979.8
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4.5	12.0
Trading Securities, Unrealized Holding Gain	0.0	0.1
Trading Securities, Unrealized Holding Loss	(0.1)	0.0
Net foreign currency gains (losses)	0.2	0.0
Fixed maturity investments, at fair value	4.6	12.1
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,472.1	2,101.7
Trading Securities, Unrealized Holding Gain	7.6	4.3
Trading Securities, Unrealized Holding Loss	(4.4)	(20.4)
Net foreign currency gains (losses)	14.7	2.1
Total fixed maturity investments including assets held for sale	$ 1,490.0	2,087.7
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities debt reclassified to assets held-for-sale		$ (6.6)	$ (8.3)	$ (9.5)