Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	$ 1,237,178	$ 1,391,721
Gross unrealized gains	114	474
Gross unrealized losses	(3,944)	(2,561)
Available-for-sale securities	1,233,348	1,389,634
Short-term marketable securities	460,088	519,642
Long-term marketable securities	773,260	869,992
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost		39
Gross unrealized gains		0
Gross unrealized losses		0
Available-for-sale securities		39
Short-term marketable securities		0
Long-term marketable securities		39
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	2,491	10,487
Gross unrealized gains	0	0
Gross unrealized losses	(4)	(2)
Available-for-sale securities	2,487	10,485
Short-term marketable securities	2,487	10,485
Long-term marketable securities	0	0
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	995,100	1,077,387
Gross unrealized gains	73	454
Gross unrealized losses	(3,365)	(2,132)
Available-for-sale securities	991,808	1,075,709
Short-term marketable securities	432,585	424,777
Long-term marketable securities	559,223	650,932
U.S. government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, cost	239,587	303,808
Gross unrealized gains	41	20
Gross unrealized losses	(575)	(427)
Available-for-sale securities	239,053	303,401
Short-term marketable securities	25,016	84,380
Long-term marketable securities	$ 214,037	$ 219,021

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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