Marketable Securities - Summary of Proceeds from Sales and Maturities of Marketable Securities and Gross Realized Gains and Losses (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investments Debt And Equity Securities [Abstract]
Proceeds from maturities	$ 4,650,000	$ 2,575,000	$ 5,350,000
Proceeds from sales	85	150,118	12,138
Gross realized gains			784
Gross realized losses	$ (1)	$ (6)	$ (5)