Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Allowance for Loan Losses
Beginning balance	$ 89,805	$ 77,634	$ 83,842	$ 68,081
Total provision	15,558	1,014	32,176	40,173
Net charge-offs:
Charge-offs	(13,757)	(654)	(23,618)	(1,297)
Recoveries	1,780	0	3,168
Net charge-offs	(11,977)	(654)	(20,450)
Loan sales	(1,520)	(17,467)	(3,702)	(46,430)
Ending Balance	91,866	60,527	91,866	60,527
Allowance:
Ending balance: individually evaluated for impairment	32,446	1,037	32,446	1,037
Ending balance: collectively evaluated for impairment	59,420	59,490	59,420	59,490
Loans:
Ending balance: individually evaluated for impairment	187,143	4,508	187,143	4,508
Ending balance: collectively evaluated for impairment	10,304,670	8,838,393	10,304,670	8,838,393
FFELP Loans
Allowance for Loan Losses
Beginning balance	4,569	6,181	5,268	6,318
Total provision	466	685	901	1,191
Net charge-offs:
Charge-offs	(479)	(654)	(1,613)	(1,297)
Recoveries	0	0	0
Net charge-offs	(479)	(654)	(1,613)
Loan sales	0	0	0	0
Ending Balance	4,556	6,212	4,556	6,212
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	4,556	6,212	4,556	6,212
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,178,876	$ 1,360,107	$ 1,178,876	$ 1,360,107
Net charge-offs as a percentage of average loans in repayment (annualized)	0.22%	0.07%	0.37%	0.13%
Allowance as a percentage of the ending total loan balance	0.39%	0.46%	0.39%	0.46%
Allowance as a percentage of the ending loans in repayment	0.54%	0.66%	0.54%	0.66%
Allowance coverage of net charge-offs (annualized)	2.38	2.40	1.41	2.40
Ending total loans, gross	$ 1,178,876	$ 1,360,107	$ 1,178,876	$ 1,360,107
Average loans in repayment	861,453	973,894	880,953	994,290
Ending loans in repayment	836,545	947,972	836,545	947,972
Private Education Loans
Allowance for Loan Losses
Beginning balance	85,236	71,453	78,574	61,763
Total provision	15,092	329	31,275	38,982
Net charge-offs:
Charge-offs	(13,278)	0	(22,005)	0
Recoveries	1,780	0	3,168
Net charge-offs	(11,498)	0	(18,837)
Loan sales	(1,520)	(17,467)	(3,702)	(46,430)
Ending Balance	87,310	54,315	87,310	54,315
Allowance:
Ending balance: individually evaluated for impairment	32,446	1,037	32,446	1,037
Ending balance: collectively evaluated for impairment	54,864	53,278	54,864	53,278
Loans:
Ending balance: individually evaluated for impairment	187,143	4,508	187,143	4,508
Ending balance: collectively evaluated for impairment	$ 9,125,794	$ 7,478,286	$ 9,125,794	$ 7,478,286
Net charge-offs as a percentage of average loans in repayment (annualized)	0.81%	0.00%	0.66%	0.00%
Allowance as a percentage of the ending total loan balance	0.94%	0.73%	0.94%	0.73%
Allowance as a percentage of the ending loans in repayment	1.54%	1.23%	1.54%	1.23%
Allowance coverage of net charge-offs (annualized)	1.90	0.00	2.32	0.00
Ending total loans, gross	$ 9,312,937	$ 7,482,794	$ 9,312,937	$ 7,482,794
Average loans in repayment	5,712,559	4,322,356	5,667,912	4,354,878
Ending loans in repayment	$ 5,666,645	$ 4,425,573	$ 5,666,645	$ 4,425,573