Derivative Financial Instruments - Cross-currency Interest Rate Swaps (Details) $ in Thousands, € in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 EUR (€)	Oct. 25, 2017 USD ($)	Dec. 31, 2006 EUR (€)
Derivative Instruments, Gain (Loss) [Line Items]
Change in fair value of cross currency interest rate swap	$ 26,379	$ 59,895	$ (15,150)
Currency Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	450,000			€ 352.7	$ 450,000
Currency Swap And Student Loan Asset Backed Euro Notes [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total impact to consolidated statements of income - (expense) income	(11,392)	9,895	(1,394)
Currency Swap And Student Loan Asset Backed Euro Notes [Member] | Currency Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Change in fair value of cross currency interest rate swap	34,208	(1,954)	(45,195)
Student Loan Asset Backed Securities Euro Note [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Issue price | €						€ 352.7
Student Loan Asset Backed Securities Euro Note [Member] | Currency Swap And Student Loan Asset Backed Euro Notes [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Re-measurement of Euro Notes	$ (45,600)	$ 11,849	$ 43,801