Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - Successor [Member] - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2018	Sep. 30, 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	$ 185	$ (47)	$ 339
Loss Reserves Ceded To Reinsurers	25,326	46,023	40,658	$ 30,767
Policyholder Benefits and Claims Incurred, Ceded	123	21,189
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(55,453)	$ 3,954,332	4,073,144	$ 3,665,271
Loss and loss expense reserves	1,868,484		4,745,015
Subrogation recoverable	(1,898,611)		(631,213)
Claim liability reported on Balance Sheet, before reinsurance	(30,127)		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	2,278,763		2,855,010
Subrogation recoverable	182,865		102,171
Claim liability reported on Balance Sheet, before reinsurance	2,461,628		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	(305,024)		(1,054,113)
Subrogation recoverable	(2,081,476)		(1,520,530)
Claim liability reported on Balance Sheet, before reinsurance	(2,386,500)		(2,574,643)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(105,255)		(135,502)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (105,255)		$ (135,502)

Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - Successor [Member] - USD ($)
$ in Thousands

9 Months Ended

Sep. 30, 2018

Sep. 30, 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

$ 185

$ (47)

$ 339

Loss Reserves Ceded To Reinsurers

25,326