Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Allowance for Loan Losses
Beginning balance	$ 91,866	$ 60,527	$ 83,842	$ 68,081
Total provision	27,497	14,898	59,673	55,071
Net charge-offs:
Charge-offs	(14,650)	(5,139)	(38,269)	(6,436)
Recoveries	1,361	0	4,529
Net charge-offs	(13,289)	(5,139)	(33,740)
Loan sales	(1,871)	(4,571)	(5,572)	(51,001)
Ending Balance	104,203	65,715	104,203	65,715
Allowance:
Ending balance: individually evaluated for impairment	43,001	2,966	43,001	2,966
Ending balance: collectively evaluated for impairment	61,202	62,749	61,202	62,749
Loans:
Ending balance: individually evaluated for impairment	231,286	13,115	231,286	13,115
Ending balance: collectively evaluated for impairment	11,752,570	9,134,268	11,752,570	9,134,268
FFELP Loans
Allowance for Loan Losses
Beginning balance	4,556	6,212	5,268	6,318
Total provision	143	291	1,044	1,482
Net charge-offs:
Charge-offs	(529)	(761)	(2,142)	(2,058)
Recoveries	0	0	0
Net charge-offs	(529)	(761)	(2,142)
Loan sales	0	0	0	0
Ending Balance	4,170	5,742	4,170	5,742
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	4,170	5,742	4,170	5,742
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,143,595	$ 1,317,963	$ 1,143,595	$ 1,317,963
Net charge-offs as a percentage of average loans in repayment (annualized)	0.25%	0.32%	0.33%	0.28%
Allowance as a percentage of the ending total loan balance	0.36%	0.44%	0.36%	0.44%
Allowance as a percentage of the ending loans in repayment	0.50%	0.61%	0.50%	0.61%
Allowance coverage of net charge-offs (annualized)	1.97	1.89	1.46	2.09
Ending total loans, gross	$ 1,143,595	$ 1,317,963	$ 1,143,595	$ 1,317,963
Average loans in repayment	839,090	953,620	868,649	980,733
Ending loans in repayment	836,585	945,230	836,585	945,230
Private Education Loans
Allowance for Loan Losses
Beginning balance	87,310	54,315	78,574	61,763
Total provision	27,354	14,607	58,629	53,589
Net charge-offs:
Charge-offs	(14,121)	(4,378)	(36,127)	(4,378)
Recoveries	1,361	0	4,529
Net charge-offs	(12,760)	(4,378)	(31,598)
Loan sales	(1,871)	(4,571)	(5,572)	(51,001)
Ending Balance	100,033	59,973	100,033	59,973
Allowance:
Ending balance: individually evaluated for impairment	43,001	2,966	43,001	2,966
Ending balance: collectively evaluated for impairment	57,032	57,007	57,032	57,007
Loans:
Ending balance: individually evaluated for impairment	231,286	13,115	231,286	13,115
Ending balance: collectively evaluated for impairment	$ 10,608,975	$ 7,816,305	$ 10,608,975	$ 7,816,305
Net charge-offs as a percentage of average loans in repayment (annualized)	0.83%	0.39%	0.72%	0.13%
Allowance as a percentage of the ending total loan balance	0.92%	0.77%	0.92%	0.77%
Allowance as a percentage of the ending loans in repayment	1.50%	1.31%	1.50%	1.31%
Allowance coverage of net charge-offs (annualized)	1.96	3.42	2.37	10.27
Ending total loans, gross	$ 10,840,261	$ 7,829,420	$ 10,840,261	$ 7,829,420
Average loans in repayment	6,118,678	4,453,775	5,848,345	4,408,852
Ending loans in repayment	$ 6,657,228	$ 4,575,143	$ 6,657,228	$ 4,575,143