Loans and Allowance for Loan Losses - Loans in Consumer Portfolio Segment Based on Refreshed LTV ratios (Details) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Jun. 30, 2018	Dec. 31, 2017
Consumer Loans
Credit quality of consumer loans
Percentage of total loans categorized by LTV ratio		100.00%	100.00%
Consumer
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 39,419	$ 37,636
Consumer | Less than or Equal to 80 Percent
Credit quality of consumer loans
Loans categorized by LTV ratio		38,603	36,524
Consumer | Greater than 80 and Less than 100 Percent
Credit quality of consumer loans
Loans categorized by LTV ratio		741	1,019
Consumer | Greater than or Equal to 100 Percent
Credit quality of consumer loans
Loans categorized by LTV ratio		42	28
Consumer | No LTV Available
Credit quality of consumer loans
Loans categorized by LTV ratio	[1]	33	65
Consumer | Residential mortgage
Credit quality of consumer loans
Loans categorized by LTV ratio		37,167	35,279
Consumer | Home equity loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 2,252	$ 2,357
Less than or Equal to 80 Percent | Consumer Loans
Credit quality of consumer loans
Percentage of total loans categorized by LTV ratio		98.00%	97.00%
Less than or Equal to 80 Percent | Home Equity Line of Credit [Member] | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 1,988	$ 2,052
Less than or Equal to 80 Percent | Residential mortgage | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 36,615	$ 34,472
Greater than 80 and Less than 100 Percent | Consumer Loans
Credit quality of consumer loans
Percentage of total loans categorized by LTV ratio		2.00%	3.00%
Greater than 80 and Less than 100 Percent | Home Equity Line of Credit [Member] | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 219	$ 248
Greater than 80 and Less than 100 Percent | Residential mortgage | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 522	$ 771
Greater than or Equal to 100 Percent | Consumer Loans
Credit quality of consumer loans
Percentage of total loans categorized by LTV ratio		0.00%	0.00%
Greater than or Equal to 100 Percent | Home Equity Line of Credit [Member] | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 15	$ 24
Greater than or Equal to 100 Percent | Residential mortgage | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 27	$ 4
No LTV Available | Consumer Loans
Credit quality of consumer loans
Percentage of total loans categorized by LTV ratio		0.00%	0.00%
No LTV Available | Home Equity Line of Credit [Member] | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 30	$ 33
No LTV Available | Residential mortgage | Consumer Loans
Credit quality of consumer loans
Loans categorized by LTV ratio		$ 3	$ 32

[1] Represents loans for which management was not able to obtain refreshed property values.