Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,131.5	$ 2,630.2
U.S. Government and agency obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	96.6	160.4
Trading Securities, Unrealized Holding Gain	0.4	0.0
Trading Securities, Unrealized Holding Loss	0.0	(0.4)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	97.0	160.0
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1,212.2	1,001.0
Trading Securities, Unrealized Holding Gain	18.7	4.3
Trading Securities, Unrealized Holding Loss	(1.5)	(5.3)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	1,229.4	1,000.0
Municipal obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	306.3	227.8
Trading Securities, Unrealized Holding Gain	6.2	2.2
Trading Securities, Unrealized Holding Loss	(0.2)	(1.2)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	312.3	228.8
Mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	2,406.6	1,170.6
Trading Securities, Unrealized Holding Gain	9.6	2.0
Trading Securities, Unrealized Holding Loss	(1.7)	(5.6)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	2,414.5	1,167.0
Foreign government, agency and provincial obligations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	1.0	1.0
Trading Securities, Unrealized Holding Gain	0.2	0.2
Trading Securities, Unrealized Holding Loss	0.0	0.0
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	1.2	1.2
Preferred Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	78.3	78.3
Trading Securities, Unrealized Holding Gain	7.1	4.4
Trading Securities, Unrealized Holding Loss	0.0	0.0
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	85.4	82.7
Fixed Income Investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	4,101.0	2,639.1
Trading Securities, Unrealized Holding Gain	42.2	13.1
Trading Securities, Unrealized Holding Loss	(3.4)	(12.5)
Net foreign currency gains (losses)	0.0	0.0
Total fixed maturity investments including assets held for sale	4,139.8	2,639.7
Star & Shield Insurance Exchange
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities Debt Reclassified to Assets Held-for-sale	$ 8.3	$ 9.5	$ 9.7	$ 10.1