Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Gross realized gains	$ 2,246	$ 3,196	$ 2,756	$ 6,396
Gross realized losses	(418)	(1,120)	(793)	(1,329)
Realized gains (losses), net	1,828	$ 2,076	1,963	$ 5,067
Unrealized loss position, total, fair value	35,586		35,586		$ 58,272
Fair Value
Less than 12 Months	30,894		30,894
12 Months or Greater	4,692		4,692
Gross Unrealized Losses
Less than 12 Months	(3,944)		(3,944)
12 Months or Greater	(154)		(154)
Total	(4,098)		(4,098)		(27,495)
Corporate securities
Unrealized loss position, total, fair value	22,387		22,387		39,869
Fair Value
Less than 12 Months	22,387		22,387
12 Months or Greater	0		0
Gross Unrealized Losses
Less than 12 Months	(2,532)		(2,532)
12 Months or Greater	0		0
Total	(2,532)		(2,532)		(23,732)
Corporate obligations
Unrealized loss position, total, fair value	13,199		13,199		13,530
Fair Value
Less than 12 Months	8,507		8,507
12 Months or Greater	4,692		4,692
Gross Unrealized Losses
Less than 12 Months	(1,412)		(1,412)
12 Months or Greater	(154)		(154)
Total	$ (1,566)		$ (1,566)		(3,441)
Mutual funds
Unrealized loss position, total, fair value					4,873
Gross Unrealized Losses
Total					$ (322)