Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Feb. 12, 2018	Dec. 31, 2017
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
satisfaction and discharge of segregated account deferred amounts				$ 3,000,158
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 253	$ 778	$ 510
Loss Reserves Ceded To Reinsurers	26,962	38,293	22,623		$ 40,658
Policyholder Benefits and Claims Incurred, Ceded	6,067	(870)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(445,523)	$ 142,853	(129,505)		$ 4,073,144
Loss and loss expense reserves	1,494,106		1,826,078
Subrogation recoverable	(1,984,826)		(1,932,960)
Claim liability reported on Balance Sheet, before reinsurance	(490,720)		(106,882)
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	1,828,116		2,246,335
Subrogation recoverable	157,307		175,694
Claim liability reported on Balance Sheet, before reinsurance	1,985,423		2,422,029
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	(273,597)		(313,595)
Subrogation recoverable	(2,142,133)		(2,108,654)
Claim liability reported on Balance Sheet, before reinsurance	(2,415,730)		(2,422,249)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(60,413)		(106,662)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (60,413)		$ (106,662)