Loans and Allowance for Loan Losses - Loans in Commercial Portfolio Segment Monitored for Credit Quality Based on Regulatory Risk Ratings (Details) $ in Millions	Dec. 31, 2019 USD ($)
Commercial
Credit quality of consumer loans
Total loans	$ 45,745
Commercial | Pass
Credit quality of consumer loans
Total loans	44,178
Commercial | Special Mention
Credit quality of consumer loans
Total loans	800
Commercial | Classified
Credit quality of consumer loans
Total loans	767
Commercial | Commercial and industrial
Credit quality of consumer loans
Total loans	27,339
Commercial | Commercial mortgage
Credit quality of consumer loans
Total loans	16,895
Commercial | Construction
Credit quality of consumer loans
Total loans	1,511
Commercial mortgage | Commercial Loans | Pass
Credit quality of consumer loans
Total loans	16,569
Commercial mortgage | Commercial Loans | Special Mention
Credit quality of consumer loans
Total loans	114
Commercial mortgage | Commercial Loans | Classified
Credit quality of consumer loans
Total loans	212
Commercial and industrial | Commercial Loans | Pass
Credit quality of consumer loans
Total loans	26,210
Commercial and industrial | Commercial Loans | Special Mention
Credit quality of consumer loans
Total loans	636
Commercial and industrial | Commercial Loans | Classified
Credit quality of consumer loans
Total loans	493
Construction | Commercial Loans | Pass
Credit quality of consumer loans
Total loans	1,399
Construction | Commercial Loans | Special Mention
Credit quality of consumer loans
Total loans	50
Construction | Commercial Loans | Classified
Credit quality of consumer loans
Total loans	$ 62