Accounting Policies and Basis of Presentation - Summary of Significant Accounting Policies (Details) shares in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) branch vote reportablesegment state	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Number of branches | branch	542
Number of states in which entity operates | state	19
Number of reportable segments | reportablesegment	1
Non-marketable securities	$ 11,600,000		$ 12,500,000
FHLB restricted stock	45,400,000		43,000,000.0
Amortization Method Qualified Affordable Housing Project Investments	$ 163,900,000		167,800,000
Allowance for credit loss, forecast period	2 years
Financing receivable, allowance for credit loss, expected, COVID-19	$ 0
Financing receivable, collateral dependent, threshold for loans individually evaluated for impairment	500,000
Reserve for unfunded commitments	12,800,000
Goodwill, impairment loss	0
Increase (decrease) in allowance for credit loss	224,314,000		225,141,000	$ 223,712,000	$ 221,893,000
Retained earnings	3,867,252,000		3,658,197,000
Debt securities, held-to-maturity, allowance for credit loss	0
Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Increase (decrease) in allowance for credit loss		$ (37,900,000)	$ (37,924,000)
Cumulative Effect, Period of Adoption, Adjustment | Cumulative effect of adoption of ASC 326
Property, Plant and Equipment [Line Items]
Reserve for unfunded commitments		8,900,000
Increase (decrease) in allowance for credit loss		(37,900,000)
Retained earnings		36,900,000
Debt securities, held-to-maturity, allowance for credit loss		0
Cumulative Effect, Period of Adoption, Adjustment | Cumulative effect of adoption of ASC 326 | Non-PCD Loans
Property, Plant and Equipment [Line Items]
Increase (decrease) in allowance for credit loss		(56,900,000)
Cumulative Effect, Period of Adoption, Adjustment | Cumulative effect of adoption of ASC 326 | PCD Loans
Property, Plant and Equipment [Line Items]
Increase (decrease) in allowance for credit loss		$ 19,000,000.0
Visa Class B Common Stock
Property, Plant and Equipment [Line Items]
Investment owned (in shares) | shares			354
Investment owned, fair value			$ 0
Held-to-maturity Securities
Property, Plant and Equipment [Line Items]
Allowance for credit loss, expected	$ 0
Class A Common Stock
Property, Plant and Equipment [Line Items]
Votes per share of common stock | vote	1
Class B Common Stock
Property, Plant and Equipment [Line Items]
Votes per share of common stock | vote	16
Minimum
Property, Plant and Equipment [Line Items]
Useful life (years)	5 years
Operating lease renewal term (years)	1 year
Maximum
Property, Plant and Equipment [Line Items]
Useful life (years)	12 years
Operating lease renewal term (years)	25 years