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, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ (47)	$ 143	$ (349)
Loss Reserves Ceded To Reinsurers	46,023	24,298	30,767	$ 44,059
Policyholder Benefits and Claims Incurred, Ceded	21,189	(11,990)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,954,332	$ 3,479,616	3,665,271	$ 2,814,754
Loss and loss expense reserves	4,704,285		4,380,769
Subrogation recoverable	(703,930)		(684,731)
Claim liability reported on Balance Sheet, before reinsurance	4,000,355		3,696,038
Unpaid Claims-Claims [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	2,392,734		2,411,105
Subrogation recoverable	615,373		583,042
Claim liability reported on Balance Sheet, before reinsurance	3,008,107		2,994,147
Unpaid Claims-Accrued Interest [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	630,338		529,703
Subrogation recoverable	163,749		132,139
Claim liability reported on Balance Sheet, before reinsurance	794,087		661,842
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	2,900,693		2,681,198
Subrogation recoverable	105,253		68,419
Claim liability reported on Balance Sheet, before reinsurance	3,005,946		2,749,617
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	(1,078,173)		(1,098,096)
Subrogation recoverable	(1,588,305)		(1,468,331)
Claim liability reported on Balance Sheet, before reinsurance	(2,666,478)		(2,566,427)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(141,307)		(143,141)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (141,307)		$ (143,141)