Municipal Bond Guarantee Insurance - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross written premiums	$ 63.2	$ 38.6	$ 25.9
Assumed (ceded) written premiums	0.0	0.0	0.0
Net written premiums	$ 63.2	$ 38.6	$ 25.9