Loans and Allowance for Credit Losses - Textual (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2020 USD ($) loan	Sep. 30, 2021 USD ($) loan	Sep. 30, 2020 USD ($) loan	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 10,825,227,000		$ 10,825,227,000		$ 12,900,897,000
Premiums and discounts associated with acquired loans			29,400,000		57,300,000
Financing receivable, accrued interest excluded from amortized costs	39,200,000		39,200,000		54,400,000
Nonaccrual loans with no allowance for credit loss	20,200,000		$ 20,200,000		16,800,000
Number of loan modifications | loan			1	5
Balance prior to TDR			$ 784,000	$ 1,900,000
Foreclosed assets and other real estate owned	11,759,000		11,759,000		18,393,000
Financing receivable, collateral dependent, amount	42,091,000		42,091,000
Number of energy credits | loan		4
Unfunded commitments, allowance for credit loss	22,400,000		22,400,000		22,400,000
Off-balance sheet, credit loss, liability, credit loss expense (reversal)	0		$ 0	$ (8,000,000)
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, term of loan			1 year
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, term of loan			3 years
Extended Maturity and Payment Deferral | COVID-19 Pandemic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loan modifications | loan			35
Troubled debt, balance	82,000,000		$ 82,000,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in process of foreclosure, amount	2,900,000		2,900,000		7,200,000
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	0		0
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	1,600,000		1,600,000		3,200,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	8,079,343,000		$ 8,079,343,000		9,223,791,000
Number of loan modifications | loan			1	5
Balance prior to TDR			$ 784,000	$ 1,948,000
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]
Loans	1,540,701,000		1,540,701,000		1,880,673,000
Number of loan modifications | loan				5
Balance prior to TDR				$ 1,948,000
Allowance for credit losses on loans				$ 16,600
Financing receivable, collateral dependent, amount	2,921,000		2,921,000
Single family residential | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	0		$ 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]
Loans	5,308,902,000		$ 5,308,902,000		5,746,863,000
Number of loan modifications | loan			1
Balance prior to TDR			$ 784,000
Allowance for credit losses on loans	5,100		5,100
Financing receivable, collateral dependent, amount	31,672,000		31,672,000
Other commercial | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	0		0
Charge-offs		$ 32,600,000
Paycheck Protection Program Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 212,100,000		$ 212,100,000		$ 904,700,000