Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Accum. Other Comprehensive Loss	Retained Income
Balance at Beginning of Year at Dec. 31, 2018	$ 15,362	$ 269	$ 2,216	$ (563)	$ 13,440
Comprehensive income:
Net income	677				677
Other comprehensive income	3			3
Total comprehensive income	680
Dividends on common stock,	(230)				(230)
Share repurchases	(500)	(3)	(22)		(475)
Stock-based compensation	19	1	19		(1)
Balance at End of Period at Mar. 31, 2019	$ 15,331	267	2,213	(560)	13,411
Comprehensive income:
Common stock, dividends, per share, declared (in dollars per share)	$ 0.86
Balance at Beginning of Year at Dec. 31, 2018	$ 15,362	269	2,216	(563)	13,440
Comprehensive income:
Net income	1,399
Other comprehensive income	6
Total comprehensive income	1,405
Share repurchases	(1,100)
Balance at End of Period at Jun. 30, 2019	15,311	265	2,226	(557)	13,377
Balance at Beginning of Year at Mar. 31, 2019	15,331	267	2,213	(560)	13,411
Comprehensive income:
Net income	722				722
Other comprehensive income	3			3
Total comprehensive income	725
Dividends on common stock,	(228)				(228)
Share repurchases	(550)	(2)	(22)		(526)
Stock-based compensation	33		35		(2)
Balance at End of Period at Jun. 30, 2019	$ 15,311	265	2,226	(557)	13,377
Comprehensive income:
Common stock, dividends, per share, declared (in dollars per share)	$ 0.86
Balance at Beginning of Year at Dec. 31, 2019	$ 15,184	259	2,209	(491)	13,207
Comprehensive income:
Net income	381				381
Other comprehensive income	10			10
Total comprehensive income	391
Dividends on common stock,	(242)				(242)
Share repurchases	(466)	(2)	(21)		(443)
Stock-based compensation	17	1	17		(1)
Balance at End of Period at Mar. 31, 2020	$ 14,884	258	2,205	(481)	12,902
Comprehensive income:
Common stock, dividends, per share, declared (in dollars per share)	$ 0.94
Balance at Beginning of Year at Dec. 31, 2019	$ 15,184	259	2,209	(491)	13,207
Comprehensive income:
Net income	773
Other comprehensive income	16
Total comprehensive income	789
Share repurchases	(669)
Balance at End of Period at Jun. 30, 2020	14,861	256	2,217	(475)	12,863
Balance at Beginning of Year at Mar. 31, 2020	14,884	258	2,205	(481)	12,902
Comprehensive income:
Net income	392				392
Other comprehensive income	6			6
Total comprehensive income	398
Dividends on common stock,	(240)				(240)
Share repurchases	(203)	(2)	(10)		(191)
Stock-based compensation	22		22
Balance at End of Period at Jun. 30, 2020	$ 14,861	$ 256	$ 2,217	$ (475)	$ 12,863
Comprehensive income:
Common stock, dividends, per share, declared (in dollars per share)	$ 0.94