Loans Receivable and Allowance for Loan Losses - Activity in the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses - balance				$ 51,842				$ 50,498	$ 51,842	$ 50,498	$ 48,900
Provision for loan losses	$ 3,750	$ 6,700	$ 3,000	1,000	$ 3,000	$ 6,000	$ 2,000	2,500	14,450	13,500	10,150
Charge-offs									(13,070)	(14,020)	(14,144)
Allowance for loan losses - balance	54,590				51,842				54,590	51,842	50,498
Consumer Portfolio Segment, Federally Insured [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses - balance				37,268				35,490	37,268	35,490
Provision for loan losses									13,000	14,000	8,000
Charge-offs									(11,562)	(12,292)	(11,730)
Purchase (sale) of financing receivables, net									0	70	50
Allowance for loan losses - balance	38,706				37,268				38,706	37,268	35,490
Consumer Portfolio Segment, Private Education Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses - balance				14,574				15,008	14,574	15,008
Provision for loan losses									(2,000)	(500)	2,150
Charge-offs									(1,313)	(1,728)	(2,414)
Recoveries - private education loans									768	954	1,050
Purchase (sale) of financing receivables, net									0	480	(140)
Transfer from repurchase obligation related to private education loans repurchased, net (a)									600	360	4,632
Allowance for loan losses - balance	12,629				14,574				12,629	14,574	15,008
Consumer Portfolio Segment, Consumer Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for loan losses - balance				$ 0				$ 0	0	0
Provision for loan losses									3,450	0	0
Charge-offs									(195)	0	0
Allowance for loan losses - balance	$ 3,255				0				$ 3,255	0	$ 0
Non-Federally Insured Loans Sold Subject to Repurchase Agreement [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Days delinquent to trigger repurchase range, minimum									60 days
Days delinquent to trigger repurchase range, maximum									90 days
Obligation to Repurchase Receivables Sold [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Loans sold under repurchase obligation agreement										39,500
Accrual for estimated loss related to repurchase of loans sold					$ 2,300					$ 2,300