Long-Term Debt (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Jul. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 01, 2013	May 19, 2011
Debt Instrument [Line Items]
Secured Debt		$ 73,993				$ 102,403				$ 73,993	$ 102,403
Junior surplus note of Ambac Assurance Segregated Account													$ 350,000
Secured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 73,993				102,986				$ 73,993	102,986
5.1% Surplus Notes, General Account, Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		10.50%								10.50%
Debt Instrument, Repurchased Face Amount		$ 108,134				11,804				$ 108,134	11,804
Net realized gains (losses) on extinguishment of debt										3,603	1,677
Debt Instrument, Face Amount		$ 754,811				862,945				$ 754,811	862,945
5.1% Surplus Notes, Segregated Account, Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		10.50%								10.50%
Debt Instrument, Repurchased Face Amount		$ 39,102								$ 39,102
Net realized gains (losses) on extinguishment of debt										212
Debt Instrument, Face Amount		0				$ 39,102				0	$ 39,102
5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage						19.50%					19.50%
Debt Instrument, Face Amount		370,237				$ 374,036				370,237	$ 374,036
One State Street [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument Principal Amount Subject To Repurchase														$ 13,056
Debt Instrument, Face Amount		20,237				$ 24,037				20,237	24,037
Extinguishment of Debt, Amount										3,799	$ 4,002
Variable Interest Entity, Not Primary Beneficiary, Aggregated Disclosure [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage						8.40%					8.40%
Successor [Member]
Debt Instrument [Line Items]
Long-term debt		991,696				$ 1,114,405				991,696	$ 1,114,405
Net realized gains (losses) on extinguishment of debt		0	$ 0	$ 2,179	$ 2,741	0	$ 24	$ 3,586	$ 1,235	4,920	4,845	$ 81
Successor [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Long-term debt		991,696				1,114,405				991,696	1,114,405
Secured Debt		73,993				102,403				73,993	102,403
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, General Account, Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt		668,667				730,648				668,667	730,648
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Surplus Notes, Segregated Account, Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt		0				33,107				0	33,107
Successor [Member] | Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt		249,036				$ 248,247				249,036	$ 248,247
Successor [Member] | Variable Interest Entities [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						6.65%					6.65%
Long-term debt		$ 12,160,544				$ 11,155,936				$ 12,160,544	$ 11,155,936
Successor [Member] | 5.1% Surplus Notes, General Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%				5.10%				5.10%	5.10%
Debt Instruments Maturity Year										2020	2020
Successor [Member] | 5.1% Surplus Notes, Segregated Account, Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%				5.10%				5.10%	5.10%
Debt Instruments Maturity Year										2020	2020
Successor [Member] | 5.1% Junior Surplus Notes, Segregated Account Due 2020 [Member] | Ambac Assurance Corporation [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.10%				5.10%				5.10%	5.10%
Debt Instruments Maturity Year										2020	2020
Consolidated Variable Interest Entities [Member]
Debt Instrument [Line Items]
Unpaid Principal Amount Of Fixed Rate Debt Accounted For Under Fair Value Option		$ 9,387,884				$ 8,854,530				$ 9,387,884	$ 8,854,530
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months		141,327								141,327
Long-term Debt, Maturities, Repayments of Principal in Year Two		307,915								307,915
Long-term Debt, Maturities, Repayments of Principal in Year Three		44,100								44,100
Long-term Debt, Maturities, Repayments of Principal in Year Four		94,024								94,024
Long-term Debt, Maturities, Repayments of Principal in Year Five		$ 0								$ 0
Consolidated Variable Interest Entities [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		0.96%				0.82%				0.96%	0.82%
Consolidated Variable Interest Entities [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage		8.35%				13.00%				8.35%	13.00%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Interest Rate on Securities Issued by Delaware Trust	2.80%