Marketable Securities (Details) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, realized gain	$ 500,000		$ 500,000
Available for sale securities, realized loss	0		0
Amortized Cost Basis		760,756,000		548,322,000
Unrealized Gains		7,205,000		392,000
Unrealized (Losses)		(369,000)		(447,000)
Total fair value of available-for-sale marketable securities		767,592,000		548,267,000
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Maturities within one year		216,774,000		158,376,000
Maturities after one year through five years		538,647,000		383,410,000
Maturities after five years through ten years		4,043,000		4,138,000
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value		1,154,000		1,177,000
Total		760,618,000		547,101,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		164,180,000		239,812,000
Fair Value - 12 Months or Greater		0		0
Fair Value - Total		164,180,000		239,812,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(369,000)		(447,000)
Unrealized Loss - 12 Months or Greater		0		0
Unrealized Loss - Total		(369,000)		(447,000)
U.S. government and government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		52,071,000		107,493,000
Unrealized Gains		97,000		55,000
Unrealized (Losses)		0		(27,000)
Total fair value of available-for-sale marketable securities		52,168,000		107,521,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months				49,241,000
Fair Value - Total				49,241,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months				(27,000)
Unrealized Loss - Total				(27,000)
Corporate bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		647,052,000		369,321,000
Unrealized Gains		1,108,000		233,000
Unrealized (Losses)		(368,000)		(361,000)
Total fair value of available-for-sale marketable securities		647,792,000		369,193,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		157,970,000		176,140,000
Fair Value - Total		157,970,000		176,140,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(368,000)		(361,000)
Unrealized Loss - Total		(368,000)		(361,000)
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis				23,891,000
Unrealized Gains				53,000
Unrealized (Losses)				0
Total fair value of available-for-sale marketable securities				23,944,000
Municipal bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		46,336,000		36,935,000
Unrealized Gains		190,000		45,000
Unrealized (Losses)		0		(59,000)
Total fair value of available-for-sale marketable securities		46,526,000		36,921,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months				14,431,000
Fair Value - Total				14,431,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months				(59,000)
Unrealized Loss - Total				(59,000)
International government agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		6,211,000		2,007,000
Unrealized Gains		0		1,000
Unrealized (Losses)		(1,000)		0
Total fair value of available-for-sale marketable securities		6,210,000		2,008,000
Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value - Less than 12 Months		6,210,000
Fair Value - Total		6,210,000
Continuous Unrealized Loss Position, Aggregate Losses [Abstract]
Unrealized Loss - Less than 12 months		(1,000)
Unrealized Loss - Total		(1,000)
Certificates of Deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		7,920,000		7,509,000
Unrealized Gains		2,000		5,000
Unrealized (Losses)		0		0
Total fair value of available-for-sale marketable securities		7,922,000		7,514,000
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost Basis		1,166,000		1,166,000
Unrealized Gains		5,808,000		0
Unrealized (Losses)		0		0
Total fair value of available-for-sale marketable securities		$ 6,974,000		$ 1,166,000