Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Gross realized gains	$ 682	$ 1,914	$ 7,078	$ 5,779
Gross realized losses	(1,683)	(51)	(3,012)	(3,945)
Realized gains (losses), net	(1,001)	1,863	4,066	1,834
Unrealized loss position less than 12 months, fair value					26,086
Unrealized loss position, less than 12 month, gross unrealized losses					(5,033)
Unrealized loss position, 12 months or greater, fair value					803
Unrealized loss position, 12 month or greater, gross unrealized losses					(451)
Unrealized loss position, total, fair value	41,282		41,282		26,889
Unrealized loss position, total, gross unrealized losses			(2,954)		(5,484)
Corporate securities
Unrealized loss position less than 12 months, fair value					15,609
Unrealized loss position, less than 12 month, gross unrealized losses					(4,757)
Unrealized loss position, 12 months or greater, fair value					803
Unrealized loss position, 12 month or greater, gross unrealized losses					(451)
Unrealized loss position, total, fair value	28,611		28,611		16,412
Unrealized loss position, total, gross unrealized losses			(2,675)		(5,208)
Corporate obligations
Unrealized loss position less than 12 months, fair value					10,477
Unrealized loss position, less than 12 month, gross unrealized losses					(276)
Unrealized loss position, 12 months or greater, fair value					0
Unrealized loss position, 12 month or greater, gross unrealized losses					0
Unrealized loss position, total, fair value	7,592		7,592		10,477
Unrealized loss position, total, gross unrealized losses			(163)		(276)
Mutual funds
Unrealized loss position, total, fair value	5,079		5,079
Unrealized loss position, total, gross unrealized losses			$ (116)