Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Composition of marketable securities
Available-for-sale Securities, Cost	$ 295,638	$ 308,175
Available-for-sale Securities, Gross Unrealized Gains	102	87
Available-for-sale Securities, Gross Unrealized Losses	(65)	(22)
Available-for-sale Securities, Fair Value	295,675	308,240
Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	72,399	85,402
Available-for-sale Securities, Gross Unrealized Gains	20	23
Available-for-sale Securities, Gross Unrealized Losses	(22)	(2)
Available-for-sale Securities, Fair Value	72,397	85,423
Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	143,425	138,368
Available-for-sale Securities, Gross Unrealized Gains	39	38
Available-for-sale Securities, Gross Unrealized Losses	(15)	(1)
Available-for-sale Securities, Fair Value	143,449	138,405
Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	79,814	84,405
Available-for-sale Securities, Gross Unrealized Gains	43	26
Available-for-sale Securities, Gross Unrealized Losses	(28)	(19)
Available-for-sale Securities, Fair Value	79,829	84,412
Certificates of deposit [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	833	998
Available-for-sale Securities, Gross Unrealized Gains	0	0
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	833	998
Certificates of deposit [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	283
Available-for-sale Securities, Gross Unrealized Gains	0
Available-for-sale Securities, Gross Unrealized Losses	0
Available-for-sale Securities, Fair Value	283
Corporate notes [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	71,611	19,169
Available-for-sale Securities, Gross Unrealized Gains	23	3
Available-for-sale Securities, Gross Unrealized Losses	(6)	(1)
Available-for-sale Securities, Fair Value	71,628	19,171
Corporate notes [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	32,309	23,293
Available-for-sale Securities, Gross Unrealized Gains	23	0
Available-for-sale Securities, Gross Unrealized Losses	(14)	(17)
Available-for-sale Securities, Fair Value	32,318	23,276
Commercial paper [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	19,973	9,995
Available-for-sale Securities, Gross Unrealized Gains	0	2
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	19,973	9,997
U.S. government treasury securities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	43,274	31,784
Available-for-sale Securities, Gross Unrealized Gains	16	5
Available-for-sale Securities, Gross Unrealized Losses	(6)	(1)
Available-for-sale Securities, Fair Value	43,284	31,788
U.S. government treasury securities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	7,603	17,055
Available-for-sale Securities, Gross Unrealized Gains	2	6
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	7,605	17,061
U.S. government treasury securities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	1,500	7,619
Available-for-sale Securities, Gross Unrealized Gains	1	4
Available-for-sale Securities, Gross Unrealized Losses	0	0
Available-for-sale Securities, Fair Value	1,501	7,623
Securities of government-sponsored entities [Member] | Restricted Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	29,125	53,618
Available-for-sale Securities, Gross Unrealized Gains	4	18
Available-for-sale Securities, Gross Unrealized Losses	(16)	(1)
Available-for-sale Securities, Fair Value	29,113	53,635
Securities of government-sponsored entities [Member] | Short-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	43,405	91,151
Available-for-sale Securities, Gross Unrealized Gains	14	27
Available-for-sale Securities, Gross Unrealized Losses	(9)	0
Available-for-sale Securities, Fair Value	43,410	91,178
Securities of government-sponsored entities [Member] | Long-term Investments [Member]
Composition of marketable securities
Available-for-sale Securities, Cost	45,722	53,493
Available-for-sale Securities, Gross Unrealized Gains	19	22
Available-for-sale Securities, Gross Unrealized Losses	(14)	(2)
Available-for-sale Securities, Fair Value	$ 45,727	$ 53,513
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	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] | 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	2 years