Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities:
Net income	$ 581,391	$ 469,014	$ 1,160,513
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Reduction of tax indemnification receivable	2,816	5,231	10,445
Amortization of unsecured debt upfront fees	2,869	2,634	2,415
Amortization of discount on unsecured borrowings	571	571	0
Acquisition related costs	952	2,603	0
Increase in other assets	(38,902)	(23,472)	(123,268)
Increase (decrease) in other liabilities	(27,807)	(6,473)	2,801
Total adjustments	(726,028)	(464,022)	(1,210,035)
Total net cash (used in) provided by operating activities	(144,637)	4,992	(49,522)
Cash flows from investing activities:
Purchase of subsidiary, net of cash acquired	(14,654)	(127,654)	0
Total net cash (used in) provided by investing activities	(12,460)	1,077,392	2,604,673
Cash flows from financing activities:
Unsecured debt issued	0	0	492,135
Unsecured debt repaid	0	0	(202,784)
Common stock dividends paid	(101,233)	(112,961)	(60,462)
Common stock repurchased	(350,264)	(713,197)	(1,530,683)
Net cash used in financing activities	(316,232)	(854,892)	(2,619,516)
Net increase (decrease) in cash, cash equivalents and restricted cash	(473,329)	227,492	(64,365)
Cash and cash equivalents at beginning of year	4,616,117	4,334,603
Cash and cash equivalents at end of year	4,149,838	4,616,117	4,334,603
Parent Company
Cash flows from operating activities:
Net income	581,391	469,014	1,160,513
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Undistributed earnings of subsidiaries	(669,624)	(555,022)	(1,271,236)
Dividends received from Sallie Mae Bank	550,000	699,500	1,444,500
Reduction of tax indemnification receivable	2,816	5,231	10,445
Amortization of unsecured debt upfront fees	2,643	2,651	2,663
Loss on early extinguishment of unsecured debt	0	0	2,784
(Increase) decrease in investment in subsidiaries, net	35,654	(9,179)	34,935
(Increase) decrease in due from subsidiaries, net	36,864	5,124	(58,310)
Increase in other assets	(13,422)	(20,533)	(16,964)
Increase (decrease) in income taxes payable, net	490	(8,713)	36,657
Decrease in payable due to entity that is a subsidiary of Navient	0	(101)	(8,430)
Increase (decrease) in other liabilities	(3,442)	(1,836)	2,165
Total adjustments	(56,498)	120,296	179,209
Total net cash (used in) provided by operating activities	524,893	589,310	1,339,722
Cash flows from investing activities:
Total net cash (used in) provided by investing activities	(14,654)	(127,654)	0
Cash flows from financing activities:
Issuance costs for unsecured debt offering	0	(375)	(1,540)
Unsecured debt issued	0	0	492,135
Unsecured debt repaid	0	0	(202,784)
Common stock dividends paid	(101,233)	(112,961)	(60,462)
Preferred stock dividends paid	(17,705)	(9,029)	(4,736)
Common stock repurchased	(350,264)	(713,197)	(1,530,683)
Net cash used in financing activities	(469,202)	(835,562)	(1,308,070)
Net increase (decrease) in cash, cash equivalents and restricted cash	41,037	(373,906)	31,652
Cash and cash equivalents at beginning of year	196,820	570,726	539,074
Cash and cash equivalents at end of year	$ 237,857	$ 196,820	$ 570,726