Accumulated Other Comprehensive Income (Loss) by Component Schedule of Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			$ 12,244
Other comprehensive income (loss) before reclassifications	$ (149)	$ 77	(770)	$ 322
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	3	2	3	42
Other comprehensive income (loss) after tax (expense) benefit	(152)	75	(773)	280
Total stockholder's equity at end of period	11,510	12,169	11,510	12,169
Cumulative effect on retained earnings, tax			8
Reclassification from AOCI, tax	(1)	1	(1)	(17)
Tax (expense) benefit on other comprehensive income (loss)	41	(21)	194	(113)
Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	20	26	30	30
Other comprehensive income (loss) before reclassifications	(1)	1	(12)	0
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	0	0	(1)	3
Other comprehensive income (loss) after tax (expense) benefit	(1)	1	(11)	(3)
Total stockholder's equity at end of period	19	27	19	27
Cumulative effect on retained earnings, tax			0
Reclassification from AOCI, tax	0	0	0	(1)
Tax (expense) benefit on other comprehensive income (loss)	0	0	3	1
Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	271	786	859	642
Other comprehensive income (loss) before reclassifications	(148)	35	(718)	228
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	10	12	28	61
Other comprehensive income (loss) after tax (expense) benefit	(158)	23	(746)	167
Total stockholder's equity at end of period	113	809	113	809
Cumulative effect on retained earnings, tax			8
Reclassification from AOCI, tax	(2)	(4)	(7)	(28)
Tax (expense) benefit on other comprehensive income (loss)	42	(16)	197	(102)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(758)	(635)	(775)	(647)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	(7)	(10)	(24)	(22)
Other comprehensive income (loss) after tax (expense) benefit	7	10	24	22
Total stockholder's equity at end of period	(751)	(625)	(751)	(625)
Cumulative effect on retained earnings, tax			0
Reclassification from AOCI, tax	1	5	6	12
Tax (expense) benefit on other comprehensive income (loss)	(1)	(5)	(6)	(12)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(138)	(145)	(98)	(198)
Other comprehensive income (loss) before reclassifications	0	41	(40)	94
Amounts reclassified from accumulated other comprehensive income (loss) net of tax	0	0	0	0
Other comprehensive income (loss) after tax (expense) benefit	0	41	(40)	94
Total stockholder's equity at end of period	(138)	(104)	(138)	(104)
Cumulative effect on retained earnings, tax			0
Reclassification from AOCI, tax	0	0	0	0
Tax (expense) benefit on other comprehensive income (loss)	0	0	0	0
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period	(605)	32	16	(173)
Cumulative effect adjustments from changes in accounting guidance, net of tax						$ (16)	$ 0
Total stockholder's equity at end of period	$ (757)	$ 107	(757)	107
ASU 2018-02 | Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						5
ASU 2018-02 | Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						137
ASU 2018-02 | Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						(130)
ASU 2018-02 | Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						0
ASU 2018-02 | Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax					$ 12	12
ASU 2016-01 | Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						0
ASU 2016-01 | Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						(28)
ASU 2016-01 | Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						0
ASU 2016-01 | Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						0
ASU 2016-01 | Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect adjustments from changes in accounting guidance, net of tax						$ (28)
Previously reported | Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			25
Previously reported | Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			750
Previously reported | Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			(645)
Previously reported | Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			(98)
Previously reported | Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Total stockholder's equity at beginning of period			$ 32	$ (173)