Loans Held for Investment and Allowance for Loan Losses - Impact of Adoption Narrative (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses on off-balance sheet exposures	$ 12,268	$ 10,174	$ 10,790	$ 12,268	$ 10,790	$ 8,640	$ 8,640	$ 9,795	$ 11,434
Provision for credit losses	100,000		27,000	196,000	47,000
Net charge-offs	74,100			131,845	24,594
Loans held for investment (outstanding balance)	25,609,885			25,609,885			24,736,642
Criticized
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment (outstanding balance)	1,000,000		629,100	1,000,000	629,100		584,100
Leveraged Lending
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net charge-offs	8,100	57,700	20,000
Energy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net charge-offs	62,400			99,675	$ 15,173
Loans held for investment (outstanding balance)	1,146,164			$ 1,146,164			$ 1,425,309
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	$ 100,000	$ 96,000	$ 27,000
Cumulative Effect, Period Of Adoption, Adjustment | Energy
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loans held for investment (outstanding balance)						$ 1,425,309