Financial Instruments and Fair Value Measurements - Composition of Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Composition of marketable securities
Available-for-sale Securities, Cost	$ 322,152	$ 295,638
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	119	102
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	155	65
Available-for-sale Securities, Fair Value	322,116	295,675
Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	78,323	72,399
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	47	20
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	23	22
Available-for-sale Securities, Fair Value	78,347	72,397
Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	140,833	143,425
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	53	39
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	37	15
Available-for-sale Securities, Fair Value	140,849	143,449
Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	102,996	79,814
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	19	43
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	95	28
Available-for-sale Securities, Fair Value	102,920	79,829
Certificates of deposit [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	869	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	869	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	79,198	71,611
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	20	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	35	6
Available-for-sale Securities, Fair Value	79,183	71,628
Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	57,940	32,309
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	11	23
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	59	14
Available-for-sale Securities, Fair Value	57,892	32,318
Commercial paper [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	10,000	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	10,000	19,973
U.S. government treasury securities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	47,823	43,274
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	43	16
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	2	6
Available-for-sale Securities, Fair Value	47,864	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	30,500	29,125
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	4	4
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	21	16
Available-for-sale Securities, Fair Value	30,483	29,113
Securities of government-sponsored entities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	49,266	43,405
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	31	14
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	2	9
Available-for-sale Securities, Fair Value	49,295	43,410
Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	45,056	45,722
Available-For-Sale Securities, Accumulated Gross Unrealized Gain, Before Tax	8	19
Available-For-Sale Securities, Accumulated Gross Unrealized Loss, Before Tax	36	14
Available-for-sale Securities, Fair Value	$ 45,028	$ 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