Variable Interest Entities Special Purpose Entities, including Variable Interest Entities - Summary of Assets and Liabilities (Details) $ in Thousands	Jun. 30, 2019 USD ($) Entity	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) Entity	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	$ 3,460,894
Restricted Cash	0	$ 0	$ 19,405		$ 0
Derivative assets	72,376		59,468
Other assets	190,566		137,628
Total assets	14,713,066		14,588,711
Interest Payable	407,096		375,808
Long-term debt	2,946,620		2,928,929
Derivative Liability	88,245		76,699
Other Liabilities	137,757		63,792
Liabilities	13,160,150		12,955,564
Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	3,138,714		2,737,286
Restricted Cash	28,500		999	$ 1,052
Financing Receivable, after Allowance for Credit Loss	4,288,572		4,287,664
Derivative assets	62,941		66,302
Other assets	4,790		1,058
Total assets	7,523,517		7,093,309
Interest Payable	529		556
Long-term debt	5,308,724		5,268,596
Long-term Debt, at par less amortized discount	339,359		0
Long-term debt	5,648,083		5,268,596
Derivative Liability	1,818,273		1,712,062
Other Liabilities	22		30
Liabilities	7,466,907		6,981,244
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	2,881,889		2,737,286
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	256,825		0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	2,881,889		2,737,286
Restricted Cash	983		999
Financing Receivable, after Allowance for Credit Loss	4,288,572		4,287,664
Derivative assets	62,941		66,302
Other assets	1,052		1,058
Total assets	7,235,437		7,093,309
Interest Payable	529		556
Long-term debt	5,308,724		5,268,596
Long-term Debt, at par less amortized discount	0		0
Long-term debt	5,308,724		5,268,596
Derivative Liability	1,818,273		1,712,062
Other Liabilities	22		30
Liabilities	7,127,548		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 $2,465,967 and $3,020,744)	2,881,889		2,737,286
Ambac UK [Member] | Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	0		0
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 and $3,020,744)	256,825		0
Restricted Cash	27,517		0
Financing Receivable, after Allowance for Credit Loss	0		0
Derivative assets	0		0
Other assets	3,738		0
Total assets	288,080		0
Interest Payable	0		0
Long-term debt	0		0
Long-term Debt, at par less amortized discount	339,359		0
Long-term debt	339,359		0
Derivative Liability	0		0
Other Liabilities	0		0
Liabilities	339,359		0
Ambac Assurance [Member] | Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Fixed income securities, at fair value (amortized cost of $2,465,967 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 $2,465,967 and $3,020,744)	$ 256,825		$ 0
Consolidated Entities [Member]
Number Of Consolidated Variable Interest Entities | Entity	8		7
Consolidated Entities [Member] | Ambac UK [Member]
Number Of Consolidated Variable Interest Entities | Entity	7		7
Consolidated Entities [Member] | Ambac Assurance [Member]
Number Of Consolidated Variable Interest Entities | Entity	1		0