Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 578,276	$ 487,476	$ 288,934
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Reduction of tax indemnification receivable	11,649	92,815	31,888
Amortization of unsecured debt upfront fees	1,117	1,128	1,316
Decrease in tax indemnification receivable	0	35,989	59,633
Increase in other assets	(3,091)	(64,777)	(72,646)
Decrease in income taxes payable, net	(30,191)	(79,693)	(19,687)
(Decrease) increase in other liabilities	5,386	15,204	1,304
Total adjustments	(593,917)	(590,217)	(460,093)
Total net cash used in operating activities	(15,641)	(102,741)	(171,159)
Cash flows from investing activities:
Total net cash used in investing activities	(2,306,714)	(3,254,863)	(3,160,150)
Cash flows from financing activities:
Unsecured debt issued	0	0	197,000
Issuance costs for unsecured debt offering	0	0	(1,057)
Redemption of Series A Preferred Stock	0	0	(165,000)
Common stock dividends paid	(51,114)	0	0
Preferred stock dividends paid	(16,837)	(15,640)	(15,714)
Common stock repurchased	(167,201)	0	0
Net cash provided by financing activities	5,361,220	4,403,324	2,994,974
Cash and cash equivalents at beginning of year	2,559,106	1,534,339
Cash and cash equivalents at end of year	5,563,877	2,559,106	1,534,339
Parent Company
Cash flows from operating activities:
Net income	578,276	487,476	288,934
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Undistributed earnings of subsidiaries	(611,692)	(532,741)	(320,775)
Dividends received from Sallie Mae Bank	254,000	0	0
Reduction of tax indemnification receivable	11,649	92,815	31,888
Amortization of unsecured debt upfront fees	811	809	596
Decrease in investment in subsidiaries, net	2,611	9,495	1,158
Decrease in tax indemnification receivable	0	35,989	59,633
Decrease (increase) in due from subsidiaries, net	6,254	(11,277)	(5,687)
Increase in other assets	(12,999)	(18,040)	(24,627)
Decrease in income taxes payable, net	(25,814)	(123,083)	(87,983)
Decrease in payable due to entity that is a subsidiary of Navient	(416)	(1,089)	(593)
(Decrease) increase in other liabilities	(5,796)	6,807	10,205
Total adjustments	(381,392)	(540,315)	(336,185)
Total net cash used in operating activities	196,884	(52,839)	(47,251)
Cash flows from investing activities:
Total net cash used in investing activities	0	0	0
Cash flows from financing activities:
Unsecured debt issued	0	0	197,000
Issuance costs for unsecured debt offering	0	0	(1,057)
Redemption of Series A Preferred Stock	0	0	(165,000)
Common stock dividends paid	(51,114)	0	0
Preferred stock dividends paid	(16,837)	(15,640)	(15,714)
Common stock repurchased	(167,201)	0	0
Net cash provided by financing activities	(235,152)	(15,640)	15,229
Net (decrease) increase in cash and cash equivalents	(38,268)	(68,479)	(32,022)
Cash and cash equivalents at beginning of year	191,776	260,255	292,277
Cash and cash equivalents at end of year	$ 153,508	$ 191,776	$ 260,255