Investment Securities (Debt securities issued by corporation) (Details) - USD ($)	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	$ 1,490,000,000	$ 2,081,100,000
Fair value investments	2,509,400,000	2,547,300,000	$ 2,406,900,000	$ 1,543,500,000
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	625,000,000	746,300,000
Fair value investments	625,000,000	746,300,000
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	26,700,000	37,300,000
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	119,400,000	212,800,000
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	291,900,000	335,600,000
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	165,400,000	143,200,000
B | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 21,600,000	17,400,000
Star & Shield Insurance Exchange | Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value		4.2
Fair value investments		$ 6,600,000