Investment Securities (Investment Holdings) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Investment securities
Fixed maturity investments, at fair value		$ 2,081,100,000
U.S. Government and agency obligations
Investment securities
Cost or amortized cost	$ 297,800,000	112,100,000
Gross unrealized gains	0	0
Gross unrealized losses	(1,300,000)	(1,100,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	296,500,000	111,000,000
Debt securities issued by corporations
Investment securities
Cost or amortized cost	867,600,000	752,000,000
Gross unrealized gains	2,900,000	2,300,000
Gross unrealized losses	(4,300,000)	(10,100,000)
Net foreign currency gains	14,700,000	2,100,000
Fixed maturity investments, at fair value	880,900,000	746,300,000
Mortgage and asset-backed securities
Investment securities
Cost or amortized cost	697,200,000	986,900,000
Gross unrealized gains	1,600,000	800,000
Gross unrealized losses	(4,100,000)	(7,900,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	694,700,000	979,800,000
Municipal obligations
Investment securities
Cost or amortized cost	252,000,000	238,700,000
Gross unrealized gains	3,700,000	1,100,000
Gross unrealized losses	(800,000)	(1,300,000)
Net foreign currency gains	0	0
Fixed maturity investments, at fair value	254,900,000	238,500,000
Foreign government, agency and provincial obligations
Investment securities
Cost or amortized cost	2,600,000	12,000,000
Gross unrealized gains	0	100,000
Gross unrealized losses	0	0
Net foreign currency gains	100,000	0
Fixed maturity investments, at fair value	2,700,000	12,100,000
Fixed income investments
Investment securities
Cost or amortized cost	2,117,200,000	2,101,700,000
Gross unrealized gains	8,200,000	4,300,000
Gross unrealized losses	(10,500,000)	(20,400,000)
Net foreign currency gains	14,800,000	2,100,000
Total fixed maturity investments	$ 2,129,700,000	2,087,700,000
Including Short Term Fixed
Investment securities
Duration of fixed maturities (years)	3 years 4 months 26 days
Excluding Short Term Investments
Investment securities
Duration of fixed maturities (years)	3 years 8 months 15 days
Star & Shield Insurance Exchange
Investment securities
Fixed maturity investments, at fair value		6,600,000.0
Less: Fixed maturity investments reclassified to assets held for sale		$ 6,600,000