Changes in Unrealized Gain, Net, on Marketable Securities Carrying Value (Detail) (Accumulated Net Unrealized Investment Gain (Loss), USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Accumulated Net Unrealized Investment Gain (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Unrealized gain on marketable securities at the beginning of the period	$ 7,509	$ 3,285	$ 11,711	$ 2,926
Reclassification of gain into earnings on maturity	(10,692)	(6,667)	(25,969)	(8,497)
Net unrealized gain due to changes in the fair value	5,169	8,678	16,244	10,867
Unrealized gain on marketable securities at the end of the period	$ 1,986	$ 5,296	$ 1,986	$ 5,296