Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Cash flows from operating activities:
Net income	$ 608,325	$ 581,391	$ 469,014
Adjustments to reconcile net income to net cash provided by operating activities:
Reduction of tax indemnification receivable	0	2,816	5,231
Amortization of unsecured debt upfront fees	2,359	2,869	2,634
Amortization of discount on unsecured borrowings	573	571	571
Acquisition related costs	0	952	2,603
Increase in other assets	(6,531)	(38,902)	(23,472)
Increase (decrease) in other liabilities	(15,180)	(27,807)	(6,473)
Total adjustments	(937,720)	(726,028)	(464,022)
Total net cash (used in) provided by operating activities	(329,395)	(144,637)	4,992
Cash flows from investing activities:
Purchase of subsidiary, net of cash acquired	0	(14,654)	(127,654)
Total net cash provided by (used in) investing activities	675,993	(12,460)	1,077,392
Cash flows from financing activities:
Common stock dividends paid	(99,172)	(101,233)	(112,961)
Common stock repurchased	(247,968)	(350,264)	(713,197)
Net cash provided by (used in) financing activities	228,155	(316,232)	(854,892)
Net increase (decrease) in cash, cash equivalents and restricted cash	574,753	(473,329)	227,492
Cash and cash equivalents at beginning of year	4,149,838	4,616,117
Cash and cash equivalents at end of year	4,700,366	4,149,838	4,616,117
Parent Company
Cash flows from operating activities:
Net income	608,325	581,391	469,014
Adjustments to reconcile net income to net cash provided by operating activities:
Undistributed earnings of subsidiaries	(691,985)	(669,624)	(555,022)
Dividends received from Sallie Mae Bank	569,500	550,000	699,500
Reduction of tax indemnification receivable	0	2,816	5,231
Amortization of unsecured debt upfront fees	2,647	2,643	2,651
(Increase) decrease in investment in subsidiaries, net	27,350	35,654	(9,179)
Decrease in due from subsidiaries, net	20,123	36,864	5,124
Increase in other assets	(14,265)	(13,422)	(20,533)
Increase (decrease) in income taxes payable, net	(4,261)	490	(8,713)
Decrease in payable due to entity that is a subsidiary of Navient	0	0	(101)
Increase (decrease) in other liabilities	1,053	(3,442)	(1,836)
Total adjustments	(89,265)	(56,498)	120,296
Total net cash (used in) provided by operating activities	519,060	524,893	589,310
Cash flows from investing activities:
Total net cash provided by (used in) investing activities	0	(14,654)	(127,654)
Cash flows from financing activities:
Issuance costs for unsecured debt offering	0	0	(375)
Common stock dividends paid	(99,172)	(101,233)	(112,961)
Preferred stock dividends paid	(18,296)	(17,705)	(9,029)
Common stock repurchased	(247,968)	(350,264)	(713,197)
Net cash provided by (used in) financing activities	(365,436)	(469,202)	(835,562)
Net increase (decrease) in cash, cash equivalents and restricted cash	153,624	41,037	(373,906)
Cash and cash equivalents at beginning of year	237,857	196,820	570,726
Cash and cash equivalents at end of year	$ 391,481	$ 237,857	$ 196,820