Income Taxes - Additional Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income Taxes Other Information [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%		35.00%		35.00%
Future Corporate Income Tax Rate		21.00%
Deferred Taxes Remeasurement Benefit Related to 2017 Tax Act		$ 333
Effective Income Tax Rate Reconciliation, Tax Credit, Amount	[1]	(117)		$ (86)		$ (69)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		(32)	[2]	(21)	[2]	(17)
Deferred Tax Assets, Valuation Allowance		143	[3]	80	[4]
Unrecognized Tax Benefits that Would Impact Effective Tax Rate		112
Unrecognized Tax Benefits, Interest on Income Taxes Accrued		17		8
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		10		(1)
Decrease in Unrecognized Tax Benefits is Reasonably Possible		$ 30
Minimum [Member]
Income Taxes Other Information [Line Items]
Operating Loss Carryforwards, Limitations on Use		2018
Maximum [Member]
Income Taxes Other Information [Line Items]
Operating Loss Carryforwards, Limitations on Use		2037
Adjustments for New Accounting Principle, Early Adoption [Member]
Income Taxes Other Information [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		$ 25		15
Investments [Member]
Income Taxes Other Information [Line Items]
Effective Income Tax Rate Reconciliation, Tax Credit, Amount		$ (43)		$ (2)		$ (6)

[1] Excludes income tax benefits of $43 million, $2 million, and $6 million for the years ended December 31, 2017, 2016, and 2015, respectively, related to tax credits, which were recognized as a reduction to the related investment asset.

[2] Includes excess tax benefits of $25 million and $15 million for the years ended December 31, 2017 and 2016, respectively, related to the Company's adoption of ASU 2016-09.

[3] The Company's DTAs and DTLs for December 31, 2017 were calculated using the enacted federal income rate of 21%.

[4] The Company's DTAs and DTLs for December 31, 2016 were calculated using the enacted federal income rate of 35%.