Fair Value Measurements - Carrying Amounts and Estimated Fair Values for Certain Financial Instruments (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Cash and due from banks	$ 2,764	$ 2,681	$ 2,052	$ 2,514
Other short-term investments	3,357	1,825
Held-to-matury securities	21	18
Loans and leases held for sale	1,205	607
Commercial and industrial loans	51,104	44,340
Commercial mortgage loans	10,717	6,974
Commercial construction loans	5,264	4,657
Commercial leases	3,677	3,600
Residential mortgage loans	16,777	15,504
Home equity	6,325	6,402
Indirect secured consumer loans	10,403	8,976
Credit card	2,436	2,470
Other consumer loans and leases	2,580	2,342
Portfolio loans and leases, net	108,168	94,162
Financial liabilities:
Deposits	125,392	108,835
Federal funds purchased	179	1,925
Other short-term borrowings	957	573
Long-term debt	15,784	14,426
Net Carrying Amount
Financial assets:
Cash and due from banks	2,764	2,681
Other short-term investments	3,357	1,825
Other securities	546	552
Held-to-matury securities	21	18
Loans and leases held for sale	156	63
Commercial and industrial loans	50,589	43,825
Commercial mortgage loans	10,645	6,894
Commercial construction loans	5,226	4,625
Commercial leases	3,651	3,582
Residential mortgage loans	16,509	15,244
Home equity	6,286	6,366
Indirect secured consumer loans	10,357	8,934
Credit card	2,282	2,314
Other consumer loans and leases	2,543	2,309
Unallocated Allowance For Loan And Lease Losses	(112)	(110)
Portfolio loans and leases, net	107,976	93,983
Financial liabilities:
Deposits	125,392	108,835
Federal funds purchased	179	1,925
Other short-term borrowings	957	573
Long-term debt	15,784	14,426
Total Fair Value
Financial assets:
Cash and due from banks	2,764	2,681
Other short-term investments	3,357	1,825
Other securities	546	552
Held-to-matury securities	21	18
Loans and leases held for sale	156	63
Commercial and industrial loans	51,029	44,668
Commercial mortgage loans	10,548	6,851
Commercial construction loans	5,291	4,688
Commercial leases	3,367	3,180
Residential mortgage loans	17,396	15,688
Home equity	6,575	6,719
Indirect secured consumer loans	10,219	8,717
Credit card	2,474	2,759
Other consumer loans and leases	2,686	2,428
Unallocated Allowance For Loan And Lease Losses	0	0
Portfolio loans and leases, net	109,585	95,698
Financial liabilities:
Deposits	125,370	108,782
Federal funds purchased	179	1,925
Other short-term borrowings	957	573
Long-term debt	16,616	14,732
Fair Value, Inputs, Level 1
Financial assets:
Cash and due from banks	2,764	2,681
Other short-term investments	3,357	1,825
Other securities	0	0
Held-to-matury securities	0	0
Loans and leases held for sale	0	0
Commercial and industrial loans	0	0
Commercial mortgage loans	0	0
Commercial construction loans	0	0
Commercial leases	0	0
Residential mortgage loans	0	0
Home equity	0	0
Indirect secured consumer loans	0	0
Credit card	0	0
Other consumer loans and leases	0	0
Unallocated Allowance For Loan And Lease Losses	0	0
Portfolio loans and leases, net	0	0
Financial liabilities:
Deposits	0	0
Federal funds purchased	179	1,925
Other short-term borrowings	0	0
Long-term debt	15,726	14,287
Fair Value, Inputs, Level 2
Financial assets:
Cash and due from banks	0	0
Other short-term investments	0	0
Other securities	546	552
Held-to-matury securities	0	0
Loans and leases held for sale	0	0
Commercial and industrial loans	0	0
Commercial mortgage loans	0	0
Commercial construction loans	0	0
Commercial leases	0	0
Residential mortgage loans	0	0
Home equity	0	0
Indirect secured consumer loans	0	0
Credit card	0	0
Other consumer loans and leases	0	0
Unallocated Allowance For Loan And Lease Losses	0	0
Portfolio loans and leases, net	0	0
Financial liabilities:
Deposits	125,370	108,782
Federal funds purchased	0	0
Other short-term borrowings	957	573
Long-term debt	890	445
Fair Value, Inputs, Level 3
Financial assets:
Cash and due from banks	0	0
Other short-term investments	0	0
Other securities	0	0
Held-to-matury securities	21	18
Loans and leases held for sale	156	63
Commercial and industrial loans	51,029	44,668
Commercial mortgage loans	10,548	6,851
Commercial construction loans	5,291	4,688
Commercial leases	3,367	3,180
Residential mortgage loans	17,396	15,688
Home equity	6,575	6,719
Indirect secured consumer loans	10,219	8,717
Credit card	2,474	2,759
Other consumer loans and leases	2,686	2,428
Unallocated Allowance For Loan And Lease Losses	0	0
Portfolio loans and leases, net	109,585	95,698
Financial liabilities:
Deposits	0	0
Federal funds purchased	0	0
Other short-term borrowings	0	0
Long-term debt	$ 0	$ 0

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