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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Available-for-sale Securities, Gross Realized Gains	$ 1,914	$ 42	$ 5,779	$ 138
Available-for-sale Securities, Gross Realized Losses	(51)	(35)	(3,945)	(40)
Unrealized loss position less than 12 months, fair value	23,665		23,665
Unrealized loss position, less than 12 month, gross unrealized losses			(4,010)
Unrealized loss position, 12 months or greater, fair value	176		176
Unrealized loss position, 12 month or greater, gross unrealized losses			(256)
Unrealized loss position, total, fair value	23,841		23,841		20,641
Unrealized loss position, total, gross unrealized losses			(4,266)		2,257
Available-for-sale Securities, Gross Realized Gain (Loss)	1,863	7	1,834	98
Corporate Securities [Member]
Unrealized loss position less than 12 months, fair value	22,725		22,725
Unrealized loss position, less than 12 month, gross unrealized losses			(3,968)
Unrealized loss position, 12 months or greater, fair value	176		176
Unrealized loss position, 12 month or greater, gross unrealized losses			(256)
Unrealized loss position, total, fair value	22,901		22,901		6,389
Unrealized loss position, total, gross unrealized losses			(4,224)		1,980
Corporate Obligations [Member]
Unrealized loss position less than 12 months, fair value	940		940
Unrealized loss position, less than 12 month, gross unrealized losses			(42)
Unrealized loss position, 12 months or greater, fair value	0		0
Unrealized loss position, 12 month or greater, gross unrealized losses			0
Unrealized loss position, total, fair value	940		940		14,252
Unrealized loss position, total, gross unrealized losses			$ (42)		$ 277