Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 487,476	$ 288,934	$ 250,327
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Reduction (income) on tax indemnification receivable	92,815	31,888	(12,283)
Amortization of unsecured debt upfront fees	1,128	1,316	1,229
Decrease in tax indemnification receivable	35,989	59,633	59,633
Increase in other assets	(69,777)	(72,646)	(47,442)
Decrease in income taxes payable, net	(79,693)	(19,687)	(20,647)
Increase in other liabilities	15,204	1,304	15,395
Total adjustments	(590,217)	(460,093)	(449,350)
Total net cash used in operating activities	(102,741)	(171,159)	(199,023)
Cash flows from investing activities:
Net cash (used in) provided by investing activities	(3,254,863)	(3,160,150)	(3,308,863)
Cash flows from financing activities:
Unsecured debt issued	0	197,000	0
Issuance costs for unsecured debt offering	0	(1,057)	0
Redemption of Series A Preferred Stock	0	(165,000)	0
Preferred stock dividends paid	(15,640)	(15,714)	(21,204)
Net cash (used in) provided by financing activities	4,403,324	2,994,974	3,036,197
Cash and cash equivalents at beginning of year	1,534,339	1,918,793
Cash and cash equivalents at end of year	2,559,106	1,534,339	1,918,793
Parent Company
Cash flows from operating activities:
Net income	487,476	288,934	250,327
Adjustments to reconcile net income to net cash (used in) provided by operating activities:
Undistributed earnings of subsidiaries	(532,741)	(320,775)	(265,278)
Reduction (income) on tax indemnification receivable	92,815	31,888	(12,283)
Amortization of unsecured debt upfront fees	809	596	0
Decrease in investment in subsidiaries, net	9,495	1,158	63,222
Decrease in tax indemnification receivable	35,989	59,633	59,633
Increase in due from subsidiaries, net	(11,277)	(5,687)	(10,438)
Increase in other assets	(18,040)	(24,627)	(8,972)
Decrease in income taxes payable, net	(123,083)	(87,983)	(54,175)
(Decrease) increase in payable due to entity that is a subsidiary of Navient	(1,089)	(593)	553
Increase in other liabilities	6,807	10,205	8,856
Total adjustments	(540,315)	(336,185)	(218,882)
Total net cash used in operating activities	(52,839)	(47,251)	31,445
Cash flows from investing activities:
Net cash (used in) provided by investing activities	0	0	0
Cash flows from financing activities:
Unsecured debt issued	0	197,000	0
Issuance costs for unsecured debt offering	0	(1,057)	0
Redemption of Series A Preferred Stock	0	(165,000)	0
Preferred stock dividends paid	(15,640)	(15,714)	(21,204)
Net cash (used in) provided by financing activities	(15,640)	15,229	(21,204)
Net (decrease) increase in cash and cash equivalents	(68,479)	(32,022)	10,241
Cash and cash equivalents at beginning of year	260,255	292,277	282,036
Cash and cash equivalents at end of year	$ 191,776	$ 260,255	$ 292,277