Loans and Allowance for Credit Losses - Textual (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($) loan	Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($) loan	Dec. 31, 2019 USD ($)	Mar. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 14,606,900,000		$ 14,606,900,000		$ 14,425,704,000
Premiums and discounts associated with acquired loans			82,200,000		91,600,000
Financing receivable, accrued interest excluded from amortized costs	66,900,000		66,900,000		48,900,000
Nonaccrual loans with no allowance for credit loss	$ 18,000,000.0		$ 18,000,000.0
Number of loans | loan	1	0	1	0
Balance prior to TDR	$ 147,000		$ 147,000
Allowance for credit losses on loans	231,643,000		231,643,000		68,244,000
Foreclosed assets and other real estate owned	14,111,000		14,111,000		19,121,000
Financing receivable, collateral dependent, amount	55,049,000		55,049,000
Unfunded commitments, allowance for credit loss	24,400,000		24,400,000		8,400,000
(Benefit) provision for credit losses on unfunded commitments			(8,000,000)	$ 950,000
Residential Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in process of foreclosure, amount	4,395,000		4,395,000		5,789,000
Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	21,755,000		21,755,000
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	2,321,000		2,321,000		4,458,000
Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	10,533,787,000		10,533,787,000		11,021,940,000
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	0	$ 0
Single family residential | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 2,207,087,000		$ 2,207,087,000		2,414,753,000
Number of loans | 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	5,479,000		5,479,000
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	1
Write-down of impaired loans				$ 138,000
Other commercial | Real Estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	6,316,444,000		$ 6,316,444,000		$ 6,358,514,000
Financing receivable, collateral dependent, amount	18,654,000		18,654,000
Other commercial | Real Estate | Energy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, collateral dependent, amount	$ 0		$ 0
Number of energy credits charged off | loan	4
Charge-offs	$ 32,600,000
Impaired loan, related allowance						$ 27,100,000
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			22.00%
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			10.00%
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%