Loans Held for Investment and Allowance for Loan Losses - Impact of Adoption Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Jan. 01, 2020	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on off-balance sheet exposures	$ 10,174	$ 8,640	$ 9,795	$ 8,640	$ 11,434
Provision for credit losses	96,000		20,000
Net charge-offs	57,703		4,588
Loans held for investment	24,519,195	24,736,642
Criticized
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment	675,900	584,100	602,800
Leveraged Lending
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net charge-offs	15,600	12,800	4,600
Energy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net charge-offs	37,307		$ 0
Loans held for investment	1,331,489	$ 1,425,309
Pandemic Economic Risk
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Provision for credit losses	30,000
Amount proactively downgraded	1,800,000
Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses, including off-balance sheet liabilities				9,100
Allowance for credit losses on off-balance sheet exposures				563
Provision for credit losses	$ 96,000
Cumulative Effect, Period Of Adoption, Adjustment | Energy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment				$ 1,425,309