Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Gross realized gains	$ 2,135	$ 681	$ 4,891	$ 7,077
Gross realized losses	(5,528)	(1,683)	(6,321)	(3,012)
Realized gains (losses), net	(3,393)	$ (1,002)	(1,430)	$ 4,065
Fair Value
Less than 12 Months	62,802		62,802
12 Months or Greater	3,883		3,883
Total	66,685		66,685		$ 58,272
Gross Unrealized Losses
Less than 12 Months	(7,122)		(7,122)
12 Months or Greater	(1,140)		(1,140)
Total	(8,262)		(8,262)		(27,495)
Corporate securities
Fair Value
Less than 12 Months	54,093		54,093
12 Months or Greater	178		178
Total	54,271		54,271		39,869
Gross Unrealized Losses
Less than 12 Months	(3,810)		(3,810)
12 Months or Greater	(34)		(34)
Total	(3,844)		(3,844)		(23,732)
Corporate obligations
Fair Value
Less than 12 Months	8,709		8,709
12 Months or Greater	3,705		3,705
Total	12,414		12,414		13,530
Gross Unrealized Losses
Less than 12 Months	(3,312)		(3,312)
12 Months or Greater	(1,106)		(1,106)
Total	$ (4,418)		$ (4,418)		(3,441)
Mutual funds
Fair Value
Total					4,873
Gross Unrealized Losses
Total					$ (322)