Loans Held for Investment and Allowance for Loan Losses (Allowance Roll Forward) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Balance at beginning of period	$ 118,075	$ 61,707	$ 110,162	$ 53,311
Provision for loan losses	32,733	32,271	78,913	57,708
Loans charged off	(49,817)	(21,625)	(90,701)	(40,143)
Recoveries of loans previously charged off	4,226	1,496	6,843	2,973
Allowance for loan losses at end of period	$ 105,217	$ 73,849	$ 105,217	$ 73,849