Loans and Allowance for Credit Losses - Textual (Details)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) loan	Mar. 31, 2021 USD ($) loan	Jun. 30, 2020 USD ($) loan credit	Jun. 30, 2021 USD ($) loan	Jun. 30, 2020 USD ($) loan	Dec. 31, 2020 USD ($) loan	Mar. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 11,386,352,000			$ 11,386,352,000		$ 12,900,897,000
Premiums and discounts associated with acquired loans				43,700,000		57,300,000
Financing receivable, accrued interest excluded from amortized costs	41,900,000			41,900,000		54,400,000
Nonaccrual loans with no allowance for credit loss	$ 15,700,000			$ 15,700,000		16,800,000
Number of loan modifications | loan	1		1	1	1
Balance prior to TDR	$ 784,000		$ 147,000	$ 784,000	$ 147,000
Foreclosed assets and other real estate owned	15,239,000			15,239,000		18,393,000
Financing receivable, collateral dependent, amount	74,815,000			74,815,000
Number of energy credits | credit			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		$ (5,000,000.0)	0	$ (8,000,000.0)
Extended Maturity and Payment Deferral | COVID-19 Pandemic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loan modifications | loan		43				43
Troubled debt, balance		$ 134,500,000				$ 134,500,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in process of foreclosure, amount	3,364,000			3,364,000		7,182,000
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	30,677,000			30,677,000
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	1,752,000			1,752,000		3,172,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 8,368,848,000			$ 8,368,848,000		9,223,791,000
Number of loan modifications | loan	1		1	1	1
Balance prior to TDR	$ 784,000		$ 147,000	$ 784,000	$ 147,000
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	1,608,028,000			1,608,028,000		1,880,673,000
Number of loan modifications | loan			1		1
Balance prior to TDR			$ 147,000		$ 147,000
Allowance for credit losses on loans			7,200		$ 7,200
Financing receivable, collateral dependent, amount	2,940,000			2,940,000
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	$ 5,332,655,000			$ 5,332,655,000		5,746,863,000
Number of loan modifications | loan	1			1
Balance prior to TDR	$ 784,000			$ 784,000
Allowance for credit losses on loans	5,100			5,100
Financing receivable, collateral dependent, amount	34,431,000			34,431,000
Other commercial | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for credit losses on loans							$ 27,100,000
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	$ 441,400,000			$ 441,400,000		$ 904,700,000