Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - Successor [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 90	$ (418)	$ 339
Loss Reserves Ceded To Reinsurers	38,735	34,691	40,658	$ 30,767
Policyholder Benefits and Claims Incurred, Ceded	(1,354)	4,112
Liability for Unpaid Claims and Claims Adjustment Expense, Net	205,588	$ 3,795,664	4,073,144	$ 3,665,271
Loss and loss expense reserves	2,139,101		4,745,015
Subrogation recoverable	(1,894,778)		(631,213)
Claim liability reported on Balance Sheet, before reinsurance	244,323		4,113,802
Unpaid Claims-Claims [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	0		2,411,632
Subrogation recoverable	0		615,391
Claim liability reported on Balance Sheet, before reinsurance	0		3,027,023
Unpaid Claims-Accrued Interest [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	0		667,988
Subrogation recoverable	0		171,755
Claim liability reported on Balance Sheet, before reinsurance	0		839,743
Present Value of Expected Net Cash Flows- Claims and Loss Expenses [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	2,592,778		2,855,010
Subrogation recoverable	275,425		102,171
Claim liability reported on Balance Sheet, before reinsurance	2,868,203		2,957,181
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(324,936)		(1,054,113)
Subrogation recoverable	(2,170,203)		(1,520,530)
Claim liability reported on Balance Sheet, before reinsurance	(2,495,139)		(2,574,643)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(128,741)		(135,502)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (128,741)		$ (135,502)