Municipal Bond Guarantee - Schedule of Net Written Premiums (Details) - HG/BAM - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Gross written premiums	$ 18.6	$ 12.4	$ 25.0	$ 31.1
Assumed (ceded) written premiums	0.0		0.0	0.0
Net written premiums	$ 18.6	$ 12.4	$ 25.0	$ 31.1