Marketable Securities (Tables)

9 Months Ended

Sep. 30, 2013

Investments Debt And Equity Securities [Abstract]

Summary of Available-for-sale Investments

The following tables summarize our amortized cost, gross unrealized gains and losses and fair value of our available-for-sale investments in marketable securities (in thousands):

	September 30, 2013
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	367,542	$	274	$	(9	)	$	367,807
Corporate debt securities		751,964		348		(214	)		752,098
Municipal securities		17,302		9		—			17,311
Total	$	1,136,808	$	631	$	(223	)	$	1,137,216
	December 31, 2012
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	464,517	$	303	$	(5	)	$	464,815
Corporate debt securities		795,962		524		(170	)		796,316
Municipal securities		5,234		—		(1	)		5,233
Total	$	1,265,713	$	827	$	(176	)	$	1,266,364

Fair Value of Available-for-Sale Marketable Securities by Contractual Maturities

The estimated fair value of available-for-sale marketable securities, classified by their contractual maturities was as follows (in thousands):

	September 30, 2013
Due within one year	$	711,502
After one year through three years		425,714
Total	$	1,137,216