Cash Equivalents and Investments (Summary of Available-For-Sale Securities by Contractual Maturity) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Cash and Cash Equivalents [Abstract]
Less than one year, Amortized Cost	$ 324,778	$ 168,679
Less than one year, Fair Value	326,458	168,689
Greater than one year but less than five years, Amortized Cost	0	0
Greater than one year but less than five years, Fair Value	0	0
Total, Amortized Cost	324,778	168,679
Total, Fair Value	$ 326,458	$ 168,689