Marketable Securities (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Amortized Cost
Amortized cost	$ 191,511,000	$ 133,499,000
Gross Unrealized Gains
Gross Unrealized Gains	132,000	104,000
Gross Unrealized Losses
Gross Unrealized Losses	(90,000)	(22,000)
Fair Value
Fair Value	191,553,000	133,581,000
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,200,000	900,000
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	40,198,000	55,531,000
Maturing after one year through three years	30,293,000	18,234,000
Amortized cost	70,491,000	73,765,000
Gross Unrealized Gains
Maturing in one year or less	43,000	27,000
Maturing after one year through three years	67,000	56,000
Gross Unrealized Gains	110,000	83,000
Gross Unrealized Losses
Maturing in one year or less	(5,000)	(4,000)
Maturing after one year through three years	(9,000)	(4,000)
Gross Unrealized Losses	(14,000)	(8,000)
Fair Value
Maturing in one year or less	40,236,000	55,554,000
Maturing after one year through three years	30,351,000	18,286,000
Fair Value	70,587,000	73,840,000
Corporate debt securities
Amortized Cost
Maturing in one year or less	92,510,000	38,973,000
Maturing after one year through three years	28,510,000	20,761,000
Amortized cost	121,020,000	59,734,000
Gross Unrealized Gains
Maturing in one year or less	20,000	9,000
Maturing after one year through three years	2,000	12,000
Gross Unrealized Gains	22,000	21,000
Gross Unrealized Losses
Maturing in one year or less	(34,000)	(9,000)
Maturing after one year through three years	(42,000)	(5,000)
Gross Unrealized Losses	(76,000)	(14,000)
Fair Value
Maturing in one year or less	92,496,000	38,973,000
Maturing after one year through three years	28,470,000	20,768,000
Fair Value	$ 120,966,000	$ 59,741,000