Loans and Allowance for Credit Losses - Textual (Details)	3 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 12,900,897,000	$ 14,425,704,000
Premiums and discounts associated with acquired loans		57,300,000	91,600,000
Financing receivable, accrued interest excluded from amortized costs		54,400,000	$ 48,900,000
Nonaccrual loans with no allowance for credit loss		$ 16,800,000
Number of loans | loan		5	4
Balance Prior to TDR		$ 1,948,000	$ 997,000
Allowance for credit losses on loans		$ 238,050,000	$ 68,244,000
Number of previously restructured loans paid off | loan		6	3
Restructured loans paid off		$ 837,265	$ 81,600
Financial impact on date of restructure			0
OREO secured by residential estate properties		18,393,000	19,121,000
Financing receivable, collateral dependent, amount		69,998,000
Unfunded commitments, allowance for credit loss		22,400,000	8,400,000
Off-balance sheet, credit loss, liability, credit loss expense (reversal)		(10,000,000.0)
COVID-19 Pandemic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 904,700,000
Number of PPP loans | loan		8,208
Outstanding balance of PPP loans		$ 975,600,000
Extended Maturity and Payment Deferral | COVID-19 Pandemic
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | loan		3,729
Troubled debt, balance		$ 2,990,845,000
Deferred interest on troubled debt		$ 20,200,000
Number of troubled debt, previously modified, recovered | loan		3,600
Troubled debt, previously modified, recovered		$ 2,900,000,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans which formal proceedings in process		7,182,000	5,789,000
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount		40,703,000
OREO Received in Full or Partial Satisfaction of Loans | Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
OREO secured by residential estate properties		3,172,000	4,458,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 9,223,791,000	$ 10,884,524,000
Number of loans | loan		5	4
Balance Prior to TDR		$ 1,948,000	$ 997,000
Financial impact on date of restructure		0	0
Real Estate | OREO Received in Full or Partial Satisfaction of Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance Prior to TDR		0
Single family residential | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 1,880,673,000	$ 2,442,064,000
Number of loans | loan		5	4
Balance Prior to TDR		$ 1,948,000	$ 997,000
Allowance for credit losses on loans		51,300
Financial impact on date of restructure		0	0
Financing receivable, collateral dependent, amount		6,950,000
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]
Balance Prior to TDR			2,100,000
Number of loans for which payment default | loan		1
Charge-off of loans			552,000
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total loans		$ 5,746,863,000	6,205,599,000
Balance Prior to TDR			$ 690,000
Number of loans for which payment default | loan			4
Financing receivable, collateral dependent, amount		46,444,000
Other commercial | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount		$ 40,703,000
Charge-offs	$ 32,600,000
Other commercial | Real Estate | Energy | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Moody's, S-2 Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Baseline economic forecast percentage		15.00%
Other commercial | Real Estate | Energy | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Moody's, S-3 Rating
Accounts, Notes, Loans and Financing Receivable [Line Items]
Baseline economic forecast percentage		17.00%
Other commercial | Real Estate | Energy | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Internal Investment Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Baseline economic forecast percentage		68.00%