Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Gross realized gains	$ 510	$ 3,200
Gross realized losses	(375)	(209)
Realized gains (losses), net	135	2,991
Unrealized loss position, total, fair value	59,989		58,272
Unrealized loss position, total, gross unrealized losses	(27,590)		(27,495)
Corporate securities
Unrealized loss position, total, fair value	40,326		39,869
Unrealized loss position, total, gross unrealized losses	(25,277)		(23,732)
Corporate obligations
Unrealized loss position, total, fair value	14,741		13,530
Unrealized loss position, total, gross unrealized losses	(2,041)		(3,441)
Mutual funds
Unrealized loss position, total, fair value	4,922		4,873
Unrealized loss position, total, gross unrealized losses	$ (272)		$ (322)