Investments - Portfolio of marketable securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available-for-sale securities:
Cost	$ 173,111	$ 195,958
Gross Unrealized Gains	6,984	12,675
Gross Unrealized Losses	(8,262)	(27,495)
Estimated Fair Value	171,833	181,138
Amounts classified as cash equivalents	(45,198)	(42,681)
Marketable securities cost	127,913	153,277
Marketable securities estimated fair value	126,635	138,457
Short-term deposits
Available-for-sale securities:
Cost	45,198	42,681
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	45,198	42,681
Mutual funds
Available-for-sale securities:
Cost	11,835	17,030
Gross Unrealized Gains	1,665	4,262
Gross Unrealized Losses	0	(322)
Estimated Fair Value	13,500	20,970
Corporate securities
Available-for-sale securities:
Cost	82,044	103,761
Gross Unrealized Gains	5,091	7,821
Gross Unrealized Losses	(3,844)	(23,732)
Estimated Fair Value	83,291	87,850
Corporate obligations
Available-for-sale securities:
Cost	34,034	32,486
Gross Unrealized Gains	228	592
Gross Unrealized Losses	(4,418)	(3,441)
Estimated Fair Value	$ 29,844	$ 29,637