Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) Segment state ATM Branch	Jan. 01, 2019 USD ($)
Accounting Policies [Line Items]
Number of retail branches | Branch	1,159
Number of automated teller machines | ATM	1,505
Number of states with automated teller machines | state	15
Number of business groups | Segment	2
Number of days to designate the loan as nonaccrual for commercial loan payment due period	90 days
Number of days to designate commercial loans will be charged off in full or charged down to the fair value of the underlying collateral payment due period	180 days
Number of days to designate the loan as nonaccrual for consumer payment due period	120 days
Second lien home equity loan with associated first lien due period	120 days
Number of days to designate the charge-off policy for most consumer loans taking effect, payment due period	120 days
Number of days to designate home equity and residential mortgage loans to get charged down to the fair value of the underlying collateral payment due period	180 days
Number of days to designate charge-off policy for credit card loans and similar unsecured products taking effect, payment due period	180 days
Threshold period by which amortized cost is greater than fair value to be considered as other-than-temporary	6 months
Minimum outstanding impaired commercial loans	$ 2,500,000
Minimum percentage for an input to be significant of the total fair value of a particular asset or liability	10.00%
Maximum limit for vary of market related value	10.00%
Amortization period of stock-based compensation awards	5 years
Vesting period for compensation cost	4 years
Minimum future rental payments under noncancelable operating leases	$ 908,000,000
Stock Options [Member]
Accounting Policies [Line Items]
Options expiration years	10 years
Stock Options [Member] | One year after the grant date [Member]
Accounting Policies [Line Items]
Rate at which employee stock options granted to be exercisable	25.00%
Maximum [Member]
Accounting Policies [Line Items]
Nonperforming loans, collectively evaluated for impairment	$ 2,500,000
Maximum [Member] | Stock Options [Member]
Accounting Policies [Line Items]
Options expiration years	10 years
Scenario, Forecast [Member] | Accounting Standards Update 2016-02 [Member]
Accounting Policies [Line Items]
Operating lease, right-of-use assets		$ 700,000,000
Operating lease, lease liabilities		$ 700,000,000