Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($) charge_off	Sep. 30, 2015 USD ($) charge_off	Sep. 30, 2016 USD ($) charge_off	Sep. 30, 2015 USD ($) charge_off
Allowance for Loan Losses
Beginning balance	$ 144,925	$ 91,866	$ 112,507	$ 83,842
Total provision	40,770	27,497	116,307	59,673
Net charge-offs:
Charge-offs	(22,428)	(14,650)	(66,065)	(38,269)
Recoveries	2,973	1,361	7,098	4,529
Net charge-offs	(19,455)	(13,289)	(58,967)	(33,740)
Loan sales	(1,401)	(1,871)	(5,008)	(5,572)
Ending Balance	164,839	104,203	164,839	104,203
Allowance:
Ending balance: individually evaluated for impairment	77,521	43,001	77,521	43,001
Ending balance: collectively evaluated for impairment	87,318	61,202	87,318	61,202
Loans:
Ending balance: individually evaluated for impairment	503,632	231,286	503,632	231,286
Ending balance: collectively evaluated for impairment	14,378,559	11,752,570	14,378,559	11,752,570
FFELP Loans
Allowance for Loan Losses
Beginning balance	2,297	4,556	3,691	5,268
Total provision	268	143	(396)	1,044
Net charge-offs:
Charge-offs	(356)	(529)	(1,086)	(2,142)
Recoveries	0	0	0	0
Net charge-offs	(356)	(529)	(1,086)	(2,142)
Loan sales	0	0	0	0
Ending Balance	2,209	4,170	2,209	4,170
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	2,209	4,170	2,209	4,170
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,033,929	$ 1,143,595	$ 1,033,929	$ 1,143,595
Net charge-offs as a percentage of average loans in repayment (annualized)	0.18%	0.25%	0.18%	0.33%
Allowance as a percentage of the ending total loan balance	0.21%	0.36%	0.21%	0.36%
Allowance as a percentage of the ending loans in repayment	0.28%	0.50%	0.28%	0.50%
Allowance coverage of net charge-offs (annualized) | charge_off	1.55	1.97	1.53	1.46
Ending total loans, gross	$ 1,033,929	$ 1,143,595	$ 1,033,929	$ 1,143,595
Average loans in repayment	791,296	839,090	795,452	868,649
Ending loans in repayment	795,665	836,585	795,665	836,585
Private Education Loans
Allowance for Loan Losses
Beginning balance	142,628	87,310	108,816	78,574
Total provision	40,502	27,354	116,703	58,629
Net charge-offs:
Charge-offs	(22,072)	(14,121)	(64,979)	(36,127)
Recoveries	2,973	1,361	7,098	4,529
Net charge-offs	(19,099)	(12,760)	(57,881)	(31,598)
Loan sales	(1,401)	(1,871)	(5,008)	(5,572)
Ending Balance	162,630	100,033	162,630	100,033
Allowance:
Ending balance: individually evaluated for impairment	77,521	43,001	77,521	43,001
Ending balance: collectively evaluated for impairment	85,109	57,032	85,109	57,032
Loans:
Ending balance: individually evaluated for impairment	503,632	231,286	503,632	231,286
Ending balance: collectively evaluated for impairment	$ 13,344,630	$ 10,608,975	$ 13,344,630	$ 10,608,975
Net charge-offs as a percentage of average loans in repayment (annualized)	0.91%	0.83%	0.97%	0.72%
Allowance as a percentage of the ending total loan balance	1.17%	0.92%	1.17%	0.92%
Allowance as a percentage of the ending loans in repayment	1.83%	1.50%	1.83%	1.50%
Allowance coverage of net charge-offs (annualized) | charge_off	2.13	1.96	2.11	2.37
Ending total loans, gross	$ 13,848,262	$ 10,840,261	$ 13,848,262	$ 10,840,261
Average loans in repayment	8,420,625	6,118,678	7,952,469	5,848,345
Ending loans in repayment	$ 8,905,812	$ 6,657,228	$ 8,905,812	$ 6,657,228