Fair Value of Financial Assets and Liabilities - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
The rate used for the discount margin to determine the estimated fair value of investment in auction rate securities	1.00%	2.00%
Auction rate securities that have partial guarantees by the U.S. government, market value	$ 19,300,000
Percentage of Loans collateralized auction rate securities	FFELP guarantees at least 95% of the loans which collateralize the auction rate securities.
Short term investment	1,667,780,000	1,786,987,000
Contractual maturity period	P90D	P90D
Coupon rate used to estimate fair value of invesments - Start Range	1.00%
Coupon rate used to estimate fair value of invesments - End Range	5.00%
Discount rate used to estimate fair value of investments	1.00%
Foreign Sovereign Debt Securities
Percentage of Available For Sale Securities Debt Securities	7.00%
Average credit rating	AA+
Foreign Debt Securities
Average credit rating	AA-
Auction Rate Securities
Average credit rating	AAA
Variable Rate Demand Note
Short term investment	$ 5,700,000
Contractual maturity period	2014