Financial Instruments and Fair Value Measurements - Composition of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Composition of marketable securities
Available-for-sale Securities, Cost	$ 297,371	$ 295,638
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	132	102
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	107	65
Available-for-sale Securities, Fair Value	297,396	295,675
Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	68,564	72,399
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	35	20
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	20	22
Available-for-sale Securities, Fair Value	68,579	72,397
Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	176,578	143,425
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	68	39
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	38	15
Available-for-sale Securities, Fair Value	176,608	143,449
Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	52,229	79,814
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	29	43
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	49	28
Available-for-sale Securities, Fair Value	52,209	79,829
Certificates of deposit [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	832	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	832	833
Certificates of deposit [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	283	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	283	283
Corporate notes [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	95,983	71,611
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	32	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	23	6
Available-for-sale Securities, Fair Value	95,992	71,628
Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	32,842	32,309
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	11	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	45	14
Available-for-sale Securities, Fair Value	32,808	32,318
Commercial paper [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	19,985	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	19,985	19,973
U.S. government treasury securities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	40,456	43,274
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	30	16
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	2	6
Available-for-sale Securities, Fair Value	40,484	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	28,108	29,125
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	5	4
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	18	16
Available-for-sale Securities, Fair Value	28,095	29,113
Securities of government-sponsored entities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	58,278	43,405
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	34	14
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	15	9
Available-for-sale Securities, Fair Value	58,297	43,410
Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	19,104	45,722
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	18	19
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	4	14
Available-for-sale Securities, Fair Value	$ 19,118	$ 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