Loans and Allowance for Credit Losses - Nonperforming Assets (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment		$ 10,567,060	$ 10,340,946	$ 10,118,798
Other real estate owned		$ 2,005	$ 1,805
Nonperforming assets to total loans and other real estate		0.39%	0.18%
Nonperforming Financial Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment		$ 38,883	$ 17,151
Repossessed assets		670
Other real estate owned		2,005	1,805
Total nonperforming assets		41,558	18,956
Nonperforming Financial Loans [Member] | Finance Receivable Nonaccrual Status [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment	[1]	37,289	13,147
Nonperforming Financial Loans [Member] | Financing Receivables, Equal to Greater than 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment		$ 1,594	$ 4,004

[1] Includes troubled debt restructurings of $14.3 million and $51 thousand as of June 30, 2019 and December 31, 2018, respectively.