Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ 1,160,513	$ 880,690	$ 578,276
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Reduction of tax indemnification receivable	10,445	9,066	11,649
Amortization of unsecured debt upfront fees	2,415	2,976	1,117
Gain on sale of Upromise subsidiary, net	0	(11,331)	0
Increase in other assets	(123,268)	(50,454)	(3,091)
Increase (decrease) in income taxes payable, net	72,191	(45,611)	(30,191)
Increase (decrease) in other liabilities	2,801	80,785	5,386
Total adjustments	(1,210,035)	(1,063,359)	(593,917)
Total net cash used in operating activities	(49,522)	(182,669)	(15,641)
Cash flows from investing activities:
Proceeds from the sale of Upromise subsidiary, net	0	16,922	0
Total net cash provided by (used in) investing activities	2,604,673	947,330	(2,306,714)
Cash flows from financing activities:
Unsecured debt issued	492,135	495,000	0
Unsecured debt repaid	(202,784)	0	0
Repurchase of Series B Preferred Stock	0	(68,055)	0
Common stock dividends paid	(60,462)	(46,351)	(51,114)
Common stock repurchased	(1,530,683)	(558,167)	(167,201)
Net cash (used in) provided by financing activities	(2,619,516)	(1,875,712)	5,361,220
Cash and cash equivalents at beginning of year	4,455,292	5,563,877
Cash and cash equivalents at end of year	4,334,603	4,455,292	5,563,877
Parent Company
Cash flows from operating activities:
Net income	1,160,513	880,690	578,276
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Undistributed earnings of subsidiaries	(1,271,236)	(939,024)	(611,692)
Dividends received from Sallie Mae Bank	1,444,500	579,400	254,000
Reduction of tax indemnification receivable	10,445	9,066	11,649
Amortization of unsecured debt upfront fees	2,663	1,029	811
Loss on early extinguishment of unsecured debt	2,784	0	0
Gain on sale of Upromise subsidiary, net	0	(11,331)	0
Decrease in investment in subsidiaries, net	34,935	53,698	2,611
(Increase) decrease in due from subsidiaries, net	(58,310)	(4,813)	6,254
Increase in other assets	(16,964)	(10,504)	(12,999)
Increase (decrease) in income taxes payable, net	36,657	(13,292)	(25,814)
Decrease in payable due to entity that is a subsidiary of Navient	(8,430)	(533)	(416)
Increase (decrease) in other liabilities	2,165	12,874	(5,796)
Total adjustments	179,209	(323,430)	(381,392)
Total net cash used in operating activities	1,339,722	557,260	196,884
Cash flows from investing activities:
Proceeds from the sale of Upromise subsidiary, net	0	16,922	0
Total net cash provided by (used in) investing activities	0	16,922	0
Cash flows from financing activities:
Issuance costs for collateralized borrowings and unsecured debt offering	(1,540)	(1,309)	0
Unsecured debt issued	492,135	495,000	0
Unsecured debt repaid	(202,784)	0	0
Repurchase of Series B Preferred Stock	0	(68,055)	0
Common stock dividends paid	(60,462)	(46,351)	(51,114)
Preferred stock dividends paid	(4,736)	(9,734)	(16,837)
Common stock repurchased	(1,530,683)	(558,167)	(167,201)
Net cash (used in) provided by financing activities	(1,308,070)	(188,616)	(235,152)
Net (decrease) increase in cash and cash equivalents	31,652	385,566	(38,268)
Cash and cash equivalents at beginning of year	539,074	153,508	191,776
Cash and cash equivalents at end of year	$ 570,726	$ 539,074	$ 153,508