Consolidated Statements of Changes in Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive (Loss) Income, Net [Member]	Noncontrolling Interest [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					$ 585
Balance (shares) at Dec. 31, 2016		77
Balance at Dec. 31, 2016	$ 24,463	$ 0	$ 6,390	$ 19,448	(1,511)	$ 136
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	560	0	0	556	0	4
Other comprehensive income (loss)	126	0	0	0	126	0
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	38				38
Distributions	(7)	0	0	0	0	(7)
Common stock purchases	(19)	0	(1)	(18)	0	0
Other equity transactions	(11)	$ 0	(8)	0	0	(3)
Balance (shares) at Mar. 31, 2017		77
Balance at Mar. 31, 2017	$ 25,112	$ 0	6,381	19,986	(1,385)	130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					623
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					1,085
Balance (shares) at Dec. 31, 2017	77	77
Balance at Dec. 31, 2017	$ 28,308	$ 0	6,368	22,206	(398)	132
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	517	0	0	513	0	4
Other comprehensive income (loss)	68	0	0	0	68	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	0			1,085	(1,085)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	0				0
Distributions	(9)	0	0	0	0	(9)
Common stock purchases	$ (90)	$ 0	(5)	(85)	0	0
Balance (shares) at Mar. 31, 2018	77	77
Balance at Mar. 31, 2018	$ 28,794	$ 0	$ 6,363	$ 23,719	(1,415)	$ 127
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax					$ 0