CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net income	$ 288,934	$ 250,327	$ 274,284
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Provisions for credit losses	185,765	159,405	90,055
Deferred tax benefit	(58,752)	(88,732)	(77,227)
Amortization of brokered deposit placement fee	9,372	10,133	10,510
Amortization of ABCP Facility upfront fee	1,316	1,229	2,337
Amortization of deferred loan origination costs and fees, net	8,258	5,811	3,746
Net amortization of discount on investments	2,082	2,043	1,716
Loss (income) on tax indemnification receivable	31,888	(12,283)	(5,398)
Depreciation of premises and equipment	11,171	9,592	7,437
Amortization of acquired intangibles	469	906	1,480
Stock-based compensation expense	27,899	22,944	21,598
Unrealized losses (gains) on derivative and hedging activities, net	7,248	2,263	(2,500)
Gains on sale of loans, net	0	(230)	(135,358)
Other adjustments to net income, net	5,836	3,524	(306)
Changes in operating assets and liabilities:
Net decrease in loans held for sale	0	0	55
Origination of loans held for sale	0	0	(55)
Increase in accrued interest receivable	(703,081)	(582,361)	(377,648)
Decrease (increase) in restricted cash and investments, net	4,247	(3,559)	(737)
Decrease in other interest-earning assets	27,528	5,731	17,634
Decrease in tax indemnification receivable	59,633	59,633	59,633
Increase in other assets	(72,920)	(47,162)	(18,070)
(Decrease) increase in income tax payable, net	(19,687)	(20,647)	62,953
Increase in accrued interest payable	14,304	3,736	303
(Decrease) increase in payable due to entity that is a subsidiary of Navient	(593)	553	(6,774)
Increase (decrease) in other liabilities	2,171	14,562	(14,731)
Total adjustments	(455,846)	(452,909)	(359,347)
Total net cash used in operating activities	(166,912)	(202,582)	(85,063)
Investing activities
Loans acquired and originated	(5,243,732)	(4,698,548)	(4,366,651)
Net proceeds from sales of loans held for investment	6,992	9,521	1,547,373
Proceeds from claim payments	49,146	64,869	111,580
Net decrease in loans held for investment	2,065,727	1,332,341	913,005
Increase in restricted cash and investment - variable interest entities	(52,366)	(22,178)	(22,439)
Purchases of available-for-sale securities	(78,327)	(55,767)	(64,112)
Proceeds from sales and maturities of available-for-sale securities	40,044	38,721	33,735
Total net cash used in investing activities	(3,212,516)	(3,331,041)	(1,847,509)
Financing activities
Brokered deposit placement fee	(12,200)	(4,371)	(4,098)
Net increase (decrease) increase in certificates of deposit	1,579,615	(434,740)	611,643
Net increase in other deposits	508,389	2,412,221	324,518
Issuance costs for collateralized borrowings	0	(2,090)	0
Borrowings collateralized by loans in securitization trusts - issued	1,440,127	1,775,692	620,681
Borrowings collateralized by loans in securitization - repaid	(534,905)	(187,686)	(41,976)
Borrowings under ABCP Facility	300,000	376,325	1,210,180
Issuance costs for unsecured debt offering	(1,057)	0	0
Repayment of borrowings under ABCP Facility	(300,000)	(876,500)	(710,005)
Fees paid - ABCP Facility	(1,281)	(1,450)	(2,337)
Unsecured debt issued	197,000	0	0
Redemption of Series A Preferred Stock	(165,000)	0	0
Preferred stock dividends paid	(15,714)	(21,204)	(19,595)
Net cash provided by (used in) financing activities	2,994,974	3,036,197	1,989,011
Net (decrease) increase in cash and cash equivalents	(384,454)	(497,426)	56,439
Cash and cash equivalents at beginning of year	1,918,793	2,416,219	2,359,780
Cash and cash equivalents at end of year	1,534,339	1,918,793	2,416,219
Cash disbursements made for:
Interest	269,017	169,854	111,563
Income taxes paid	282,278	271,721	205,698
Income taxes refunded	$ (1,401)	$ (86)	$ (25,151)