Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Thousands	Sep. 30, 2019 USD ($) Entity	Dec. 31, 2018 USD ($) Entity	Sep. 30, 2018 USD ($)
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	$ 3,476,694
Restricted Cash	12,561	$ 19,405	$ 0
Derivative assets	82,162	59,468
Other assets	113,189	137,628
Total assets	13,451,104	14,588,711
Interest Payable	423,815	375,808
Long-term debt	2,954,929	2,928,929
Derivative Liability	103,314	76,699
Other Liabilities	95,267	63,792
Liabilities	11,822,440	12,955,564
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	3,158,608	2,737,286
Restricted Cash	3,168	999	$ 1,024
Financing Receivable, after Allowance for Credit Loss	2,965,539	4,287,664
Derivative assets	67,921	66,302
Other assets	4,154	1,058
Total assets	6,199,390	7,093,309
Interest Payable	2,656	556
Long-term debt	4,151,433	5,268,596
Long-term Debt, at par less amortized discount	201,738	0
Long-term debt	4,353,171	5,268,596
Derivative Liability	1,779,077	1,712,062
Other Liabilities	36	30
Liabilities	6,134,940	6,981,244
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	2,995,894	2,737,286
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	162,714	0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	2,995,894	2,737,286
Restricted Cash	970	999
Financing Receivable, after Allowance for Credit Loss	2,965,539	4,287,664
Derivative assets	67,921	66,302
Other assets	3,180	1,058
Total assets	6,033,504	7,093,309
Interest Payable	2,656	556
Long-term debt	4,151,433	5,268,596
Long-term Debt, at par less amortized discount	0	0
Long-term debt	4,151,433	5,268,596
Derivative Liability	1,779,077	1,712,062
Other Liabilities	36	30
Liabilities	5,933,202	6,981,244
Ambac UK [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	2,995,894	2,737,286
Ambac UK [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	0	0
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	162,714	0
Restricted Cash	2,198	0
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	974	0
Total assets	165,886	0
Interest Payable	0	0
Long-term debt	0	0
Long-term Debt, at par less amortized discount	201,738	0
Long-term debt	201,738	0
Derivative Liability	0	0
Other Liabilities	0	0
Liabilities	201,738	0
Ambac Assurance [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	0	0
Ambac Assurance [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,561,278 and $3,020,744)	$ 162,714	$ 0
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	7	7
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	6	7
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1	0