Financial Guarantee Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 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	$ 287	$ 185	$ 510
Loss Reserves Ceded To Reinsurers	25,749	25,326	22,623		$ 40,658
Policyholder Benefits and Claims Incurred, Ceded	5,999	123
Liability for Unpaid Claims and Claims Adjustment Expense, Net	(513,554)	$ (55,453)	(129,505)		$ 4,073,144
Loss and loss expense reserves	1,522,304		1,826,078
Subrogation recoverable	(2,082,268)		(1,932,960)
Claim liability reported on Balance Sheet, before reinsurance	(559,964)		(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	1,803,743		2,246,335
Subrogation recoverable	135,599		175,694
Claim liability reported on Balance Sheet, before reinsurance	1,939,342		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	(225,967)		(313,595)
Subrogation recoverable	(2,217,867)		(2,108,654)
Claim liability reported on Balance Sheet, before reinsurance	(2,443,834)		(2,422,249)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Loss and loss expense reserves	(55,472)		(106,662)
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	$ (55,472)		$ (106,662)