Loans and Leases (Narrative) (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) payment_period	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) payment_period	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Loan	Dec. 31, 2016 Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Collateral required on Residential real estate loan, lower limit	65.00%		65.00%
Collateral required on Residential real estate loan, upper limit	80.00%		80.00%
Draw period for credit line products			10 years
Other U.S. loans and non-U.S. loans, short duration advances	$ 1,400,000,000		$ 1,400,000,000		$ 906,400,000
Demand deposits	73,400,000		73,400,000		127,600,000
Loans classified as held for sale	0		0		20,900,000
Leases classified as held for sale	25,400,000		25,400,000		33,100,000
Interest income that would have been recorded for nonperforming loans	2,000,000	$ 2,500,000	4,200,000	$ 4,800,000
Undrawn loan commitments and standby letters of credit	13,600,000		13,600,000		9,400,000
Total loans and leases	32,473,200,000		$ 32,473,200,000		32,592,200,000
Performing status, minimum period of time			6 months
Recorded investment	100,000	$ 200,000	$ 100,000	200,000
Unpaid principal balance			100,000	$ 0
Foreclosed residential real estate properties with carrying value	3,600,000		3,600,000
Consumer loans with carrying value collateralized by residential real estate property	14,400,000		14,400,000
Nonperforming Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	67,900,000		67,900,000		72,500,000
Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans and leases	$ 36,300,000		$ 36,300,000		$ 25,900,000
Re-Defaulted | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans and leases | Loan					1	1
Lower Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Days contractually past due for interest and principal and the loan is not well-secured and in the process of collection			90 days
Lower Limit | Performing Financing Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for which troubled debt restructurings with modified terms is performing before ceasing to be classified as impaired loan			6 months
Upper Limit
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of payment periods | payment_period	6		6
Limit of individual homogeneous loans collective evaluation for impairment and excluded from impaired loan disclosures (less than)	$ 1,000,000		$ 1,000,000
Equity
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equity credit lines	$ 764,200,000		$ 764,200,000		$ 908,600,000
Percentage of total equity credit lines	94.00%		94.00%		93.00%