Marketable Securities (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Amortized Cost
Amortized cost	$ 237,583,000	$ 173,014,000
Gross Unrealized Gains
Gross Unrealized Gains	118,000	88,000
Gross Unrealized Losses
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total	(98,000)	(79,000)
Fair Value
Fair Value	237,603,000	173,023,000
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,300,000	1,400,000
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	51,907,000	44,580,000
Maturing after one year through three years	8,914,000	16,108,000
Amortized cost	60,821,000	60,688,000
Gross Unrealized Gains
Maturing in one year or less	27,000	30,000
Maturing after one year through three years	38,000	49,000
Gross Unrealized Gains	65,000	79,000
Gross Unrealized Losses
Maturing in one year or less	(3,000)	(6,000)
Maturing after one year through three years		(1,000)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total	(3,000)	(7,000)
Fair Value
Maturing in one year or less	51,931,000	44,604,000
Maturing after one year through three years	8,952,000	16,156,000
Fair Value	60,883,000	60,760,000
Corporate debt securities
Amortized Cost
Maturing in one year or less	114,200,000	89,636,000
Maturing after one year through three years	62,562,000	22,690,000
Amortized cost	176,762,000	112,326,000
Gross Unrealized Gains
Maturing in one year or less	22,000	6,000
Maturing after one year through three years	31,000	3,000
Gross Unrealized Gains	53,000	9,000
Gross Unrealized Losses
Maturing in one year or less	(65,000)	(37,000)
Maturing after one year through three years	(30,000)	(35,000)
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax, Total	(95,000)	(72,000)
Fair Value
Maturing in one year or less	114,157,000	89,605,000
Maturing after one year through three years	62,563,000	22,658,000
Fair Value	$ 176,720,000	$ 112,263,000