Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Available-for-sale Securities, Gross Realized Gains	$ 2,607	$ 23	$ 3,865	$ 95
Available-for-sale Securities, Gross Realized Losses	(3,852)	(4)	(3,894)	(5)
Unrealized loss position less than 12 months, fair value	154,010		154,010
Unrealized loss position, less than 12 month, gross unrealized losses	(1,959)		(1,959)
Unrealized loss position, 12 months or greater, fair value	654		654
Unrealized loss position, 12 month or greater, gross unrealized losses	(284)		(284)
Unrealized loss position, total, fair value	154,664		154,664		68,831
Unrealized loss position, total, gross unrealized losses	2,243		2,243		(2,257)
Available-for-sale Securities, Gross Realized Gain (Loss)	(1,245)	19	(29)	90
Mutual Funds [Member]
Unrealized loss position less than 12 months, fair value	13,889		13,889
Unrealized loss position, less than 12 month, gross unrealized losses	(23)		(23)
Unrealized loss position, 12 months or greater, fair value	0		0
Unrealized loss position, 12 month or greater, gross unrealized losses	0		0
Unrealized loss position, total, fair value	13,889		13,889
Unrealized loss position, total, gross unrealized losses	23		23
US Treasury Securities [Member]
Unrealized loss position less than 12 months, fair value	49,549		49,549
Unrealized loss position, less than 12 month, gross unrealized losses	(11)		(11)
Unrealized loss position, 12 months or greater, fair value	0		0
Unrealized loss position, 12 month or greater, gross unrealized losses	0		0
Unrealized loss position, total, fair value	49,549		49,549
Unrealized loss position, total, gross unrealized losses	11		11
Corporate Securities [Member]
Unrealized loss position less than 12 months, fair value	66,474		66,474
Unrealized loss position, less than 12 month, gross unrealized losses	(1,836)		(1,836)
Unrealized loss position, 12 months or greater, fair value	654		654
Unrealized loss position, 12 month or greater, gross unrealized losses	(284)		(284)
Unrealized loss position, total, fair value	67,128		67,128		20,117
Unrealized loss position, total, gross unrealized losses	2,120		2,120		(1,980)
Corporate Obligations [Member]
Unrealized loss position less than 12 months, fair value	24,098		24,098
Unrealized loss position, less than 12 month, gross unrealized losses	(89)		(89)
Unrealized loss position, 12 months or greater, fair value	0		0
Unrealized loss position, 12 month or greater, gross unrealized losses	0		0
Unrealized loss position, total, fair value	24,098		24,098		48,714
Unrealized loss position, total, gross unrealized losses	$ 89		$ 89		$ (277)