Accumulated Other Comprehensive Income (Loss) by Component - Changes in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year			$ 11,969
Other comprehensive income (loss) before reclassifications	$ 149	$ 274	245	$ 514
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	25	8	40	(11)
Other comprehensive income (loss) after tax (expense) benefit	124	266	205	525
Balance, end of period	12,028	11,886	12,028	11,886
Reclassification from AOCI, current period, tax	(11)	(3)	(18)	8
Tax (expense) benefit on other comprehensive income (loss)	(51)	(159)	(92)	(281)
Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	26	32	30	27
Other comprehensive income (loss) before reclassifications	(1)	(1)	(1)	2
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	(1)	0	3	(2)
Other comprehensive income (loss) after tax (expense) benefit	0	(1)	(4)	4
Balance, end of period	26	31	26	31
Reclassification from AOCI, current period, tax	1	0	(1)	1
Tax (expense) benefit on other comprehensive income (loss)	0	1	1	(2)
Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	709	624	642	390
Other comprehensive income (loss) before reclassifications	108	323	193	546
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	31	13	49	2
Other comprehensive income (loss) after tax (expense) benefit	77	310	144	544
Balance, end of period	786	934	786	934
Reclassification from AOCI, current period, tax	(15)	(6)	(24)	1
Tax (expense) benefit on other comprehensive income (loss)	(48)	(157)	(86)	(273)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(640)	(642)	(647)	(648)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	(5)	(5)	(12)	(11)
Other comprehensive income (loss) after tax (expense) benefit	5	5	12	11
Balance, end of period	(635)	(637)	(635)	(637)
Reclassification from AOCI, current period, tax	3	3	7	6
Tax (expense) benefit on other comprehensive income (loss)	(3)	(3)	(7)	(6)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(187)	(70)	(198)	(84)
Other comprehensive income (loss) before reclassifications	42	(48)	53	(34)
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	0	0	0	0
Other comprehensive income (loss) after tax (expense) benefit	42	(48)	53	(34)
Balance, end of period	(145)	(118)	(145)	(118)
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	(92)	(56)	(173)	(315)
Balance, end of period	$ 32	$ 210	$ 32	$ 210