Consolidated Statements of Total Comprehensive Income - Successor [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Net premiums earned	$ 175,277	$ 197,287	$ 312,595
Net investment income:
Securities available-for-sale and short-term	337,774	281,049	249,337
Other investments	23,179	32,318	16,952
Total net investment income	360,953	313,367	266,289
Other-than-temporary impairment losses:
Total other-than-temporary impairment losses	(54,625)	(89,700)	(66,692)
Portion of other-than-temporary impairment recognized in other comprehensive income	34,454	67,881	41,033
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	(20,171)	(21,819)	(25,659)
Realized Investment Gains (Losses)	5,366	39,284	53,476
Change in fair value of credit derivatives:
Realized gains and other settlements	1,589	912	2,785
Unrealized gains (losses)	14,783	19,194	38,916
Net change in fair value of credit derivatives	16,372	20,106	41,701
Net gains (losses) on interest rate derivatives	59,565	(50,273)	(42,544)
Net realized gains (losses) on extinguishment of debt	4,920	4,845	81
Other income (expense)	(706)	17,445	7,150
Income (loss) on variable interest entities	19,670	(14,093)	31,569
Total revenues	621,246	506,149	644,658
Expenses:
Losses and loss expenses (benefit)	513,186	(11,489)	(768,707)
Insurance intangible amortization	150,854	174,608	169,557
Operating expenses	121,516	113,660	102,702
Interest expense	119,941	124,344	116,537
Goodwill, Impairment Loss	0	0	514,511
Total expenses	905,497	401,123	134,600
Pre-tax income (loss)	(284,251)	105,026	510,058
Provision for income taxes	44,464	30,709	17,364
Net income (loss)	(328,715)	74,317	492,694
Less: net gain (loss) attributable to noncontrolling interest	0	(526)	(709)
Net income (loss) attributable to common shareholders	(328,715)	74,843	493,403
Other comprehensive income (loss), after tax:
Net income (loss)	(328,715)	74,317	492,694
Unrealized gains (losses) on securities, net of deferred income taxes of $0	(81,520)	67,900	(159,730)
Gains (losses) on foreign currency translation, net of deferred income taxes of $0	73,586	(122,128)	(45,025)
Changes to postretirement benefit, net of tax of $0	1,273	23	(687)
Total other comprehensive income (loss), net of tax	(6,661)	(54,205)	(205,442)
Total comprehensive income (loss)	(335,376)	20,112	287,252
Less: comprehensive (loss) gain attributable to the noncontrolling interest:
Net gain (loss)	0	(526)	(709)
Currency translation adjustments	0	0	(374)
Total comprehensive income (loss) attributable to Ambac Financial Group, Inc.	$ (335,376)	$ 20,638	$ 288,335
Basic	$ (7.25)	$ 1.66	$ 10.92
Diluted	$ (7.25)	$ 1.64	$ 10.72