Accumulated Other Comprehensive Income (Loss) by Component - Changes in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year			$ 11,969
Other comprehensive income (loss) before reclassifications	$ 77	$ 52	322	$ 566
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	2	25	42	14
Other comprehensive income (loss) after tax (expense) benefit	75	27	280	552
Balance, end of period	12,169	12,195	12,169	12,195
Reclassification from AOCI, current period, tax	1	(12)	(17)	(4)
Tax (expense) benefit on other comprehensive income (loss)	(21)	(24)	(113)	(305)
Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	26	31	30	27
Other comprehensive income (loss) before reclassifications	1	7	0	9
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	0	4	3	2
Other comprehensive income (loss) after tax (expense) benefit	1	3	(3)	7
Balance, end of period	27	34	27	34
Reclassification from AOCI, current period, tax	0	(2)	(1)	(1)
Tax (expense) benefit on other comprehensive income (loss)	0	(2)	1	(4)
Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	786	934	642	390
Other comprehensive income (loss) before reclassifications	35	69	228	615
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	12	27	61	29
Other comprehensive income (loss) after tax (expense) benefit	23	42	167	586
Balance, end of period	809	976	809	976
Reclassification from AOCI, current period, tax	(4)	(13)	(28)	(12)
Tax (expense) benefit on other comprehensive income (loss)	(16)	(19)	(102)	(292)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(635)	(637)	(647)	(648)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	(10)	(6)	(22)	(17)
Other comprehensive income (loss) after tax (expense) benefit	10	6	22	17
Balance, end of period	(625)	(631)	(625)	(631)
Reclassification from AOCI, current period, tax	5	3	12	9
Tax (expense) benefit on other comprehensive income (loss)	(5)	(3)	(12)	(9)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(145)	(118)	(198)	(84)
Other comprehensive income (loss) before reclassifications	41	(24)	94	(58)
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	0	0	0	0
Other comprehensive income (loss) after tax (expense) benefit	41	(24)	94	(58)
Balance, end of period	(104)	(142)	(104)	(142)
Reclassification from AOCI, current period, tax	0	0	0	0
Tax (expense) benefit on other comprehensive income (loss)	0	0	0	0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	32	210	(173)	(315)
Balance, end of period	$ 107	$ 237	$ 107	$ 237