Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Balance, beginning of year at Dec. 31, 2015		$ 683	$ 2,153	$ 9,313	$ (315)	$ (78)
Stock-based compensation			10			5
Dividends paid to common stockholders				(746)
Net income	$ 618			618
Other comprehensive income	552				552
Balance, end of period at Sep. 30, 2016	12,195	683	2,163	9,185	237	(73)
Balance, beginning of year at Jun. 30, 2016					210
Net income	343
Other comprehensive income	27
Balance, end of period at Sep. 30, 2016	12,195	683	2,163	9,185	237	(73)
Balance, beginning of year at Dec. 31, 2016	11,969	683	2,173	9,359	(173)	(73)
Stock-based compensation			(6)			12
Dividends paid to common stockholders				(762)
Net income	676			676
Other comprehensive income	280				280
Balance, end of period at Sep. 30, 2017	12,169	683	2,167	9,273	107	(61)
Balance, beginning of year at Jun. 30, 2017					32
Net income	144
Other comprehensive income	75
Balance, end of period at Sep. 30, 2017	$ 12,169	$ 683	$ 2,167	$ 9,273	$ 107	$ (61)