Financial Instruments and Fair Value Measurements , Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 685,868	$ 709,030
Unrealized Gains	623	384
Unrealized Losses	(278)	(924)
Fair Value	686,213	708,490
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	217,944	356,625
Less Than 12 Months, Unrealized Losses	(249)	(907)
12 Months or Greater, Fair Value	13,142	6,011
12 Months or Greater, Unrealized Losses	(29)	(17)
Total, Fair Value	231,086	362,636
Total, Unrealized Losses	(278)	(924)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Due within one year	417,605	436,766
Due within one to three years	268,608	271,724
Fair Value	686,213	708,490
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	568,456	589,526
Unrealized Gains	593	365
Unrealized Losses	(243)	(875)
Fair Value	568,806	589,016
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	177,364	317,011
Less Than 12 Months, Unrealized Losses	(214)	(858)
12 Months or Greater, Fair Value	13,142	6,011
12 Months or Greater, Unrealized Losses	(29)	(17)
Total, Fair Value	190,506	323,022
Total, Unrealized Losses	(243)	(875)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	568,806	589,016
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,758	51,156
Unrealized Gains	1	3
Unrealized Losses	(9)	(4)
Fair Value	43,750	51,155
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	8,375	8,185
Less Than 12 Months, Unrealized Losses	(9)	(4)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	8,375	8,185
Total, Unrealized Losses	(9)	(4)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	43,750	51,155
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,825	39,745
Unrealized Gains	21	15
Unrealized Losses	(26)	(39)
Fair Value	49,820	39,721
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value	32,205	26,684
Less Than 12 Months, Unrealized Losses	(26)	(39)
12 Months or Greater, Fair Value	0	0
12 Months or Greater, Unrealized Losses	0	0
Total, Fair Value	32,205	26,684
Total, Unrealized Losses	(26)	(39)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	49,820	39,721
Certificates of deposit and term deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	13,080	22,854
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	13,080	22,854
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	13,080	22,854
U.S. government and agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,749	5,749
Unrealized Gains	8	1
Unrealized Losses	0	(6)
Fair Value	10,757	5,744
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Less Than 12 Months, Fair Value		4,745
Less Than 12 Months, Unrealized Losses		(6)
12 Months or Greater, Fair Value		0
12 Months or Greater, Unrealized Losses		0
Total, Fair Value		4,745
Total, Unrealized Losses		(6)
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Fair Value	$ 10,757	$ 5,744

Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax

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