Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 75	$ 153	$ 272	$ 470
Unrealized holding gains (losses) arising during the period on:
Investments with no other-than-temporary impairment, net of tax provision (benefit) of $(2), $30, $(31) and $53	(64)	62	(186)	106
Investments with other-than-temporary impairment, net of tax provision (benefit) of $1, $23, $(2) and $51	6	46	(5)	96
Unrealized holding gains (losses) arising during the period, net of tax	(58)	108	(191)	202
Less: reclassification adjustment for gains (losses) included in net income (loss), net of tax provision (benefit) of $0, $5, $(2) and $26	(1)	9	(4)	48
Change in net unrealized gains (losses) on investments	(57)	99	(187)	154
Net unrealized gains (losses) arising during the period on financial guaranty variable interest entities' liabilities with recourse attributable to changes in instrument-specific credit risk, net of tax provision (benefit) of $1 and $0 (see Note 1)	3	0	(1)	0
Less: reclassification adjustment for gains (losses) included in net income (loss), net of tax provision (benefit) of $0 and $(1)	(1)	0	(3)	0
Change in net unrealized gains (losses) on financial guaranty variable interest entities' liabilities with recourse	4	0	2	0
Other, net of tax provision	(9)	10	(3)	12
Other comprehensive income (loss)	(62)	109	(188)	166
Comprehensive income (loss)	$ 13	$ 262	$ 84	$ 636