Financial Instruments and Fair Value Measurements - Composition of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Composition of marketable securities
Available-for-sale Securities, Cost	$ 346,272	$ 295,638
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	111	102
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	215	65
Available-for-sale Securities, Fair Value	346,168	295,675
Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	92,614	72,399
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	44	20
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	66	22
Available-for-sale Securities, Fair Value	92,592	72,397
Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	169,833	143,425
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	52	39
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	61	15
Available-for-sale Securities, Fair Value	169,824	143,449
Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	83,825	79,814
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	15	43
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	88	28
Available-for-sale Securities, Fair Value	83,752	79,829
Certificates of deposit [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	572	833
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	0	0
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Available-for-sale Securities, Fair Value	572	833
Certificates of deposit [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	0	283
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	0	0
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Available-for-sale Securities, Fair Value	0	283
Corporate notes [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	99,429	71,611
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	18	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	53	6
Available-for-sale Securities, Fair Value	99,394	71,628
Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	45,281	32,309
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	11	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	54	14
Available-for-sale Securities, Fair Value	45,238	32,318
Commercial paper [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	2,497	19,973
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	0	0
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Available-for-sale Securities, Fair Value	2,497	19,973
U.S. government treasury securities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	57,945	43,274
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	39	16
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	13	6
Available-for-sale Securities, Fair Value	57,971	43,284
U.S. government treasury securities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	1,500	7,603
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	2	2
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	0	0
Available-for-sale Securities, Fair Value	1,502	7,605
U.S. government treasury securities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost		1,500
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax		1
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax		0
Available-for-sale Securities, Fair Value		1,501
Securities of government-sponsored entities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	34,669	29,125
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	5	4
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	53	16
Available-for-sale Securities, Fair Value	34,621	29,113
Securities of government-sponsored entities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	65,835	43,405
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	32	14
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	8	9
Available-for-sale Securities, Fair Value	65,859	43,410
Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	38,544	45,722
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	4	19
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	34	14
Available-for-sale Securities, Fair Value	$ 38,514	$ 45,727
Minimum [Member] | Certificates of deposit [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year
Minimum [Member] | Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year
Minimum [Member] | U.S. government treasury securities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year
Minimum [Member] | Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity		1 year
Maximum [Member] | Certificates of deposit [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year	1 year
Maximum [Member] | Certificates of deposit [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	2 years
Maximum [Member] | Corporate notes [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year	1 year
Maximum [Member] | Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	2 years
Maximum [Member] | Commercial paper [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year	1 year
Maximum [Member] | U.S. government treasury securities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	2 years	2 years
Maximum [Member] | U.S. government treasury securities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year	1 year
Maximum [Member] | U.S. government treasury securities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	2 years
Maximum [Member] | Securities of government-sponsored entities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	2 years	2 years
Maximum [Member] | Securities of government-sponsored entities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity	1 year	1 year
Maximum [Member] | Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities Contractual Maturity		2 years