Accumulated Other Comprehensive Income (Loss) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Balance at beginning of period	$ 3,171,000,000	$ 4,141,000,000	$ 4,330,000,000	$ 3,799,000,000
Other comprehensive income (loss) before reclassifications	14,000,000	10,000,000	36,000,000	72,000,000
Reclassification adjustments from accumulated other comprehensive income			1,000,000
Balance at end of period	3,123,000,000	4,094,000,000	3,123,000,000	4,094,000,000
Allowance for credit loss	0		0
Unrealized Gains (Losses) Available-for-Sale Securities *
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Balance at beginning of period	66,000,000	30,000,000	41,000,000	(28,000,000)
Other comprehensive income (loss) before reclassifications	12,000,000	11,000,000	38,000,000	69,000,000
Reclassification adjustments from accumulated other comprehensive income			1,000,000
Balance at end of period	78,000,000	41,000,000	78,000,000	41,000,000
Retirement Plan Liabilities Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Balance at beginning of period	3,000,000	(3,000,000)	3,000,000	(3,000,000)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Reclassification adjustments from accumulated other comprehensive income			0
Balance at end of period	3,000,000	(3,000,000)	3,000,000	(3,000,000)
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Balance at beginning of period	(4,000,000)	1,000,000	0	(3,000,000)
Other comprehensive income (loss) before reclassifications	2,000,000	(1,000,000)	(2,000,000)	3,000,000
Reclassification adjustments from accumulated other comprehensive income			0
Balance at end of period	(2,000,000)	0	(2,000,000)	0
Total Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax
Balance at beginning of period	65,000,000	28,000,000	44,000,000	(34,000,000)
Balance at end of period	$ 79,000,000	$ 38,000,000	$ 79,000,000	$ 38,000,000