Schedule of Components of Investment Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jul. 31, 2013	Jul. 31, 2012	Jul. 31, 2013	Jul. 31, 2012
Investments Debt And Equity Securities [Abstract]
Interest income	$ 3,374	$ 4,915	$ 6,771	$ 9,664
Realized gains	4,762	3,463	5,400	3,554
Realized losses	(3,749)	(1,205)	(4,430)	(1,584)
Total investment income	$ 4,387	$ 7,173	$ 7,741	$ 11,634