Income Taxes Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Income Tax Rate Reconciliation [Line Items]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount		$ 982		$ 939		$ 944
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		35.00%		35.00%		35.00%
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount		$ 66		$ 53		$ 25
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent		2.40%		2.00%		0.90%
Effective Income Tax Rate Reconciliation, Tax Exempt Income, Amount		$ (90)		$ (86)		$ (88)
Effective Income Tax Rate Reconciliation, Tax Exempt Income, Percent		3.20%		3.20%		3.30%
Effective Income Tax Rate Reconciliation, Change in UTBs, Amount		$ 26		$ 6		$ (31)
Effective Income Tax Rate Reconciliation, Change in UTBs, Percent		0.90%		0.20%		(1.10%)
Effective Income Tax Rate Reconciliation, Tax Credit, Amount	[1]	$ (117)		$ (86)		$ (69)
Effective Income Tax Rate Reconciliation, Tax Credit, Percent	[1]	4.20%		3.20%		2.60%
Effective Income Tax Rate Reconciliation, Deferred Taxes Remeasurement, Amount		$ (303)	[2]	$ 0		$ 0
Effective Income Tax Rate Reconciliation, Deferred Taxes Remeasurement, Percent		(10.80%)	[2]	(0.00%)		(0.00%)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		$ (32)	[3]	$ (21)	[3]	$ (17)
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(1.10%)	[3]	(0.80%)	[3]	(0.60%)
Income Tax Expense (Benefit)		$ 532		$ 805		$ 764
Effective Income Tax Rate Reconciliation, Percent		19.00%		30.00%		28.30%
Investments [Member]
Income Tax Rate Reconciliation [Line Items]
Effective Income Tax Rate Reconciliation, Tax Credit, Amount		$ (43)		$ (2)		$ (6)
Adjustments for New Accounting Principle, Early Adoption [Member]
Income Tax Rate Reconciliation [Line Items]
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount		$ 25		$ 15

[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 reasonable estimates as of December 31, 2017, which could be adjusted as additional analysis is completed in 2018.

[3] 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.