Fair Value Measurements Schedule of Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 913,031	$ 893,251
Total Fair Value
Cash Equivalents and Marketable Securities:
Assets, fair value	986,708	921,879
Other Assets:
Note receivable	22,879
Other Liabilities:
Contingent consideration obligation related to acquisition	(2,600)	(1,200)
Level 1
Cash Equivalents and Marketable Securities:
Assets, fair value	43,900	29,728
Other Assets:
Note receivable	0
Other Liabilities:
Contingent consideration obligation related to acquisition	0	0
Level 2
Cash Equivalents and Marketable Securities:
Assets, fair value	942,808	892,151
Other Assets:
Note receivable	0
Other Liabilities:
Contingent consideration obligation related to acquisition	0	0
Level 3
Cash Equivalents and Marketable Securities:
Assets, fair value	0	0
Other Assets:
Note receivable	22,879
Other Liabilities:
Contingent consideration obligation related to acquisition	(2,600)	(1,200)
Money market funds | Total Fair Value
Cash Equivalents and Marketable Securities:
Cash equivalents	40,482	22,255
Money market funds | Level 1
Cash Equivalents and Marketable Securities:
Cash equivalents	40,482	22,255
Money market funds | Level 2
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Money market funds | Level 3
Cash Equivalents and Marketable Securities:
Cash equivalents	0	0
Certificates of deposit
Cash Equivalents and Marketable Securities:
Available-for-sale securities	222	3,100
Certificates of deposit | Total Fair Value
Cash Equivalents and Marketable Securities:
Available-for-sale securities	3,418	7,473
Certificates of deposit | Level 1
Cash Equivalents and Marketable Securities:
Available-for-sale securities	3,418	7,473
Certificates of deposit | Level 2
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Certificates of deposit | Level 3
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Commercial paper
Cash Equivalents and Marketable Securities:
Available-for-sale securities		7,482
Commercial paper | Total Fair Value
Cash Equivalents and Marketable Securities:
Available-for-sale securities	29,999	9,482
Commercial paper | Level 1
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Commercial paper | Level 2
Cash Equivalents and Marketable Securities:
Available-for-sale securities	29,999	9,482
Commercial paper | Level 3
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
U.S. government agency obligations
Cash Equivalents and Marketable Securities:
Available-for-sale securities	174,948	189,674
U.S. government agency obligations | Total Fair Value
Cash Equivalents and Marketable Securities:
Available-for-sale securities	174,948	189,674
U.S. government agency obligations | Level 1
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
U.S. government agency obligations | Level 2
Cash Equivalents and Marketable Securities:
Available-for-sale securities	174,948	189,674
U.S. government agency obligations | Level 3
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Corporate bonds
Cash Equivalents and Marketable Securities:
Available-for-sale securities	737,861	692,995
Corporate bonds | Total Fair Value
Cash Equivalents and Marketable Securities:
Available-for-sale securities	737,861	692,995
Corporate bonds | Level 1
Cash Equivalents and Marketable Securities:
Available-for-sale securities	0	0
Corporate bonds | Level 2
Cash Equivalents and Marketable Securities:
Available-for-sale securities	737,861	692,995
Corporate bonds | Level 3
Cash Equivalents and Marketable Securities:
Available-for-sale securities	$ 0	$ 0