Investments - Fair value and gross unrealized losses of the available for sale securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Gross realized gains	$ 3,200	$ 1,258
Gross realized losses	(209)	(42)
Realized gains, net	2,991	1,216
Unrealized loss position less than 12 months, fair value	31,384		26,086
Unrealized loss position, less than 12 month, gross unrealized losses	(3,302)		(5,033)
Unrealized loss position, 12 months or greater, fair value	551		803
Unrealized loss position, 12 month or greater, gross unrealized losses	(658)		(451)
Unrealized loss position, total, fair value	31,935		26,889
Unrealized loss position, total, gross unrealized losses	(3,960)		(5,484)
Corporate securities
Unrealized loss position less than 12 months, fair value	30,339		15,609
Unrealized loss position, less than 12 month, gross unrealized losses	(3,299)		(4,757)
Unrealized loss position, 12 months or greater, fair value	551		803
Unrealized loss position, 12 month or greater, gross unrealized losses	(658)		(451)
Unrealized loss position, total, fair value	30,890		16,412
Unrealized loss position, total, gross unrealized losses	(3,957)		(5,208)
Corporate obligations
Unrealized loss position less than 12 months, fair value	1,045		10,477
Unrealized loss position, less than 12 month, gross unrealized losses	(3)		(276)
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	1,045		10,477
Unrealized loss position, total, gross unrealized losses	$ (3)		$ (276)