Fair Value (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 1,899	$ 839
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	108	0
Loans and Leases Receivable, Net Amount	121,617	117,800
Nonmarketable equity securities	1,233	1,046
Deposits	93,256	79,022
Short-term Debt	11,413	12,673
Long-term Debt	44,226	54,128
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,899	839
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	95
Loans and Leases Receivable, Net Amount	121,617	117,800
Nonmarketable equity securities	1,233	1,046
Deposits	93,256	79,022
Short-term Debt	11,413	12,673
Long-term Debt	44,226	54,128
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,865	789
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	95
Loans and Leases Receivable, Net Amount	123,302	118,750
Nonmarketable equity securities	1,239	1,067
Deposits	91,029	78,469
Short-term Debt	11,417	12,675
Long-term Debt	46,624	56,120
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0
Loans and Leases Receivable, Net Amount	0	0
Nonmarketable equity securities	0	0
Deposits	0	0
Short-term Debt	0	0
Long-term Debt	0	0
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	1,865	789
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	0
Loans and Leases Receivable, Net Amount	0	0
Nonmarketable equity securities	1,190	1,012
Deposits	0	0
Short-term Debt	0	0
Long-term Debt	27,807	22,036
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	0	0
Loans Receivable Held-for-sale, Net, Not Part of Disposal Group	95
Loans and Leases Receivable, Net Amount	123,302	118,750
Nonmarketable equity securities	49	55
Deposits	91,029	78,469
Short-term Debt	11,417	12,675
Long-term Debt	$ 18,817	$ 34,084