Investment Securities - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Pledged securities not separately reported	$ 48,000.0	$ 46,000.0
Loss on sale	113.0	5.0	$ 62.0
Available-for-sale securities transferred	496.0	4,900.0
Available-for-sale securities, gross unrealized gain	(2.8)	87.0
Net impairment losses	0.0	3.0	1.0
US Treasury and federal agencies, mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Securities sold	12,200.0
Loss on sale	39.0
Agency securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	$ 0.0	0.0	0.0
Federal family education loan program
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Asset Backed Securities, Average Remaining Term	4 years 7 months 6 days
Net impairment losses	$ 0.0	0.0	0.0
Asset-backed securities, student loans
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	0.0	0.0	0.0
Less than maximum credit exposure	70.0
Non-U.S. debt securities, mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	1.0	2.0	1.0
State and political subdivisions
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	0.0	0.0	0.0
U.S. Non-Agency Residential Mortgage-Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	0.0	0.0	0.0
U.S. Non-Agency Commercial Mortgage-Backed Securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Net impairment losses	0.0	$ 1.0	$ 0.0
Gross pre-tax unrealized losses	$ 657.0
Number of securities in loss position | security	1,283
Minimum
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Available-for-sale securities transferred to held to maturity	10 years
Maximum
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Available-for-sale securities transferred to held to maturity	42 years