Loans and Allowance for Credit Losses - Nonperforming Assets (Parenthetical) (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
Nonperforming Financial Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment		38,883	17,151
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
Troubled Debt Restructuring [Member] | Nonperforming Financial Loans [Member] | Finance Receivable Nonaccrual Status [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for investment		$ 14,300	$ 51

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