Accumulated Other Comprehensive Income (Loss) by Component - Changes in accumulated other comprehensive income (loss) by component (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	$ 11,969
Other comprehensive income (loss) before reclassifications	96	$ 240
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	15	(19)
Other comprehensive income (loss) after tax (expense) benefit	81	259
Balance, end of period	11,693	11,470
Reclassification from AOCI, current period, tax	(7)	11
Tax (expense) benefit on other comprehensive income (loss)	(41)	(122)
Net unrealized gains (losses) on investments with OTTI losses
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	30	27
Other comprehensive income (loss) before reclassifications	0	3
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	4	(2)
Other comprehensive income (loss) after tax (expense) benefit	(4)	5
Balance, end of period	26	32
Reclassification from AOCI, current period, tax	(2)	1
Tax (expense) benefit on other comprehensive income (loss)	1	(3)
Net unrealized gains (losses) on other investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	642	390
Other comprehensive income (loss) before reclassifications	85	223
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	18	(11)
Other comprehensive income (loss) after tax (expense) benefit	67	234
Balance, end of period	709	624
Reclassification from AOCI, current period, tax	(9)	7
Tax (expense) benefit on other comprehensive income (loss)	(38)	(116)
Pension and postretirement benefits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(647)	(648)
Other comprehensive income (loss) before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	(7)	(6)
Other comprehensive income (loss) after tax (expense) benefit	7	6
Balance, end of period	(640)	(642)
Reclassification from AOCI, current period, tax	4	3
Tax (expense) benefit on other comprehensive income (loss)	(4)	(3)
Cumulative foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(198)	(84)
Other comprehensive income (loss) before reclassifications	11	14
Amounts reclassified from accumulated other comprehensive income (loss) after tax (expense) benefit	0	0
Other comprehensive income (loss) after tax (expense) benefit	11	14
Balance, end of period	(187)	(70)
Reclassification from AOCI, current period, tax	0	0
Tax (expense) benefit on other comprehensive income (loss)	0	0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of year	(173)	(315)
Balance, end of period	$ (92)	$ (56)