Loans and Allowance for Credit Losses - Textual (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) loan	Mar. 31, 2020 USD ($) loan	Dec. 31, 2020 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 12,195,873,000		$ 12,900,897,000
Premiums and discounts associated with acquired loans	51,400,000		57,300,000
Financing receivable, accrued interest excluded from amortized costs	47,800,000		54,400,000
Nonaccrual loans with no allowance for credit loss	$ 18,800,000		16,800,000
Number of loan modifications | loan	0	0
Foreclosed assets and other real estate owned	$ 11,168,000		$ 18,393,000
Financing receivable, collateral dependent, amount	68,788,000
Unfunded commitments, allowance for credit loss	22,400,000
Off-balance sheet, credit loss, liability, credit loss expense (reversal)	$ 0	$ (3,000,000.0)
Extended Maturity and Payment Deferral | COVID-19 Pandemic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loan modifications | loan	3,700		3,700
Troubled debt, balance	$ 3,000,000,000.0		$ 3,000,000,000.0
Deferred interest on troubled debt	$ 5,900,000
Number of trouble debt, previously modified, recovered | loan	3,300
Troubled debt, previously modified, recovered	$ 2,600,000,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in process of foreclosure, amount	5,838,000		7,182,000
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	39,827,000
OREO Received in Full or Partial Satisfaction of Loans | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed assets and other real estate owned	1,995,000		3,172,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	8,819,907,000		9,223,791,000
Real Estate | OREO Received in Full or Partial Satisfaction of Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance prior to TDR	0	$ 0
Single family residential | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	1,730,056,000		1,880,673,000
Financing receivable, collateral dependent, amount	5,086,000
Single family residential | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	$ 0
Commercial real estate | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans with subsequent default | loan	0	0
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 5,638,010,000		5,746,863,000
Financing receivable, collateral dependent, amount	18,414,000
Other commercial | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	0
Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans	$ 797,600,000		$ 904,700,000