Parent Only Statements - Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 288,934	$ 250,327	$ 274,284
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Loss (income) on tax indemnification receivable	31,888	(12,283)	(5,398)
Amortization of unsecured debt upfront fees	1,316	1,229	2,337
Decrease in tax indemnification receivable	59,633	59,633	59,633
Increase in other assets	(72,920)	(47,162)	(18,070)
Decrease in income taxes payable, net	(19,687)	(20,647)	62,953
(Decrease) increase in payable due to entity that is a subsidiary of Navient	(593)	553	(6,774)
Increase in other liabilities	2,171	14,562	(14,731)
Total adjustments	(455,846)	(452,909)	(359,347)
Total net cash used in operating activities	(166,912)	(202,582)	(85,063)
Cash flows from investing activities:
Net cash provided by (used in) investing activities	(3,212,516)	(3,331,041)	(1,847,509)
Cash flows from financing activities:
Unsecured debt issued	197,000	0	0
Issuance costs for unsecured debt offering	(1,057)	0	0
Redemption of Series A Preferred Stock	(165,000)	0	0
Preferred stock dividends paid	(15,714)	(21,204)	(19,595)
Net cash provided by (used in) financing activities	2,994,974	3,036,197	1,989,011
Net (decrease) increase in cash and cash equivalents	(384,454)	(497,426)	56,439
Cash and cash equivalents at beginning of year	1,918,793	2,416,219	2,359,780
Cash and cash equivalents at end of year	1,534,339	1,918,793	2,416,219
Parent Company
Cash flows from operating activities:
Net income	288,934	250,327	274,284
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Undistributed earnings of subsidiaries	(320,775)	(265,278)	(295,638)
Loss (income) on tax indemnification receivable	31,888	(12,283)	(5,398)
Amortization of unsecured debt upfront fees	596	0	0
Decrease (increase) in investment in subsidiaries, net	1,158	63,222	(103,602)
Decrease in tax indemnification receivable	59,633	59,633	59,633
(Increase) decrease in due from subsidiaries, net	(5,687)	(10,438)	11,012
Increase in other assets	(24,627)	(8,972)	(14,366)
Decrease in income taxes payable, net	(87,983)	(54,175)	(53,167)
(Decrease) increase in payable due to entity that is a subsidiary of Navient	(593)	553	(6,774)
Increase in other liabilities	10,205	8,856	1,402
Total adjustments	(336,185)	(218,882)	(406,898)
Total net cash used in operating activities	(47,251)	31,445	(132,614)
Cash flows from investing activities:
Net cash provided by (used in) investing activities	0	0	0
Cash flows from financing activities:
Unsecured debt issued	197,000	0	0
Issuance costs for unsecured debt offering	(1,057)	0	0
Redemption of Series A Preferred Stock	(165,000)	0	0
Preferred stock dividends paid	(15,714)	(21,204)	(19,595)
Net cash provided by (used in) financing activities	15,229	(21,204)	(19,595)
Net (decrease) increase in cash and cash equivalents	(32,022)	10,241	(152,209)
Cash and cash equivalents at beginning of year	292,277	282,036	434,245
Cash and cash equivalents at end of year	$ 260,255	$ 292,277	$ 282,036