Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,566.9	$ 2,081.1
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	59.5	112.1
Trading Securities, Unrealized Holding Gain	0.0	0.0
Trading Securities, Unrealized Holding Loss	(0.3)	(1.1)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	59.2	111.0
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	705.4	752.0
Trading Securities, Unrealized Holding Gain	4.9	2.3
Trading Securities, Unrealized Holding Loss	(1.8)	(10.1)
Net foreign currency gains (losses)	9.5	2.1
Fixed maturity investments, at fair value	718.0	746.3
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	272.6	238.7
Trading Securities, Unrealized Holding Gain	2.6	1.1
Trading Securities, Unrealized Holding Loss	(0.8)	(1.3)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	274.4	238.5
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	514.7	986.9
Trading Securities, Unrealized Holding Gain	1.2	0.8
Trading Securities, Unrealized Holding Loss	(4.5)	(7.9)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	511.4	979.8
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	3.9	12.0
Trading Securities, Unrealized Holding Gain	0.0	0.1
Trading Securities, Unrealized Holding Loss	0.0	0.0
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	3.9	12.1
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,556.1	2,101.7
Trading Securities, Unrealized Holding Gain	8.7	4.3
Trading Securities, Unrealized Holding Loss	(7.4)	(20.4)
Net foreign currency gains (losses)	9.5	2.1
Total fixed maturity investments including assets held for sale	$ 1,566.9	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)	$ (9.2)	$ (9.5)