Loans and Allowance for Loan Losses (Narrative) (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($) contract	Sep. 30, 2019 USD ($) contract	Sep. 30, 2020 USD ($) contract	Sep. 30, 2019 USD ($) contract	Jun. 30, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized, cash basis		$ 1,300,000	$ 363,000	$ 2,700,000	$ 2,000,000.0
Loans pledged as collateral		15,290,000,000		15,290,000,000				$ 12,110,000,000
Total loans		39,648,824,000		39,648,824,000				37,187,814,000
Allowance for credit loss, reversal		6,100,000		19,400,000
Transfer of third-party lending partnership consumer loans to held for sale loans.		513,200,000		1,310,000,000
Total allowance for loan losses		603,800,000	265,013,000	603,800,000	265,013,000	$ 588,648,000	$ 281,402,000	281,402,000	$ 257,376,000	$ 250,555,000
Reserve for unfunded commitments		60,800,000		60,800,000
Allowance for credit losses		664,600,000		664,600,000			$ 282,777,000
Allowance for credit loss, increase		14,900,000		271,400,000
Provision for credit losses	[1]	43,383,000	27,562,000	$ 343,956,000	63,250,000
Allowance for Loan and Lease Losses Write-offs, Net		$ 28,500,000
Estimated impact of currently enacted stimulus and an unemployment rate for the end of year		8.00%		8.00%
ACL to loans ratio during period		0.05%
ACL to loans ratio		1.68%		1.68%
ACL to loans ratio, excluding PPP loans		1.80%		1.80%
Recorded Investment		$ 8,144,000	$ 14,621,000	$ 60,492,000	$ 31,352,000
Commitments to land additional funds to TDRs		$ 0		$ 0				0
Troubled Debt Restructuring That Subsequently Defaults
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of contracts | contract		1	3	5	4
Recorded Investment		$ 21,000	$ 321,000	$ 666,000	$ 326,000
Total commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		20,014,151,000		20,014,151,000				$ 16,769,370,000
Recorded Investment		5,758,000	8,177,000	18,346,000	15,822,000
Total commercial and industrial | Small Business Administration (SBA), CARES Act, Paycheck Protection Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		2,710,000,000		2,710,000,000
Interest Income Recorded
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized		$ 4,300,000	$ 2,500,000	$ 9,200,000	$ 8,000,000.0
Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loan substandard period (in days)				90 days
Loss and Charged Off
Accounts, Notes, Loans and Financing Receivable [Line Items]
Retail loan charge off (in days)				120 days

[1] Beginning January 1, 2020, provision calculation is based on current expected credit loss methodology. Prior to January 1, 2020, calculation was based on incurred loss methodology.