Fair Value Measurements - Schedule of Fair Value Measurement (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 1,288,138	$ 1,233,348
Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	40,920	2,487
Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	981,076	991,808
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	266,142	239,053
Level 1
Cash Equivalents and Marketable Securities:
Assets, fair value	12,748	2,664
Contingent consideration obligation related to completed acquisitions	0
Level 1 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	8,726	1,250
Level 1 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 1 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	4,022	1,414
Level 2
Cash Equivalents and Marketable Securities:
Assets, fair value	1,288,138	1,233,348
Contingent consideration obligation related to completed acquisitions	0
Level 2 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Level 2 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	40,920	2,487
Level 2 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	981,076	991,808
Level 2 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	266,142	239,053
Level 2 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3
Cash Equivalents and Marketable Securities:
Assets, fair value	0	0
Contingent consideration obligation related to completed acquisitions	(7,100)
Level 3 | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Level 3 | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Level 3 | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Total Fair Value
Cash Equivalents and Marketable Securities:
Assets, fair value	1,300,886	1,236,012
Contingent consideration obligation related to completed acquisitions	(7,100)
Total Fair Value | Money market funds
Cash Equivalents and Marketable Securities:
Cash equivalents	8,726	1,250
Total Fair Value | Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities	40,920	2,487
Total Fair Value | Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	981,076	991,808
Total Fair Value | U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	266,142	239,053
Total Fair Value | Mutual funds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 4,022	$ 1,414