Marketable Securities (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Amortized Cost
Amortized cost	$ 217,241,000	$ 133,499,000
Gross Unrealized Gains
Gross Unrealized Gains	166,000	104,000
Gross Unrealized Losses
Gross Unrealized Losses	(83,000)	(22,000)
Fair Value
Fair Value	217,324,000	133,581,000
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,300,000	900,000
U.S. government and municipal obligations
Amortized Cost
Maturing in one year or less	40,967,000	55,531,000
Maturing after one year through three years	43,248,000	18,234,000
Amortized cost	84,215,000	73,765,000
Gross Unrealized Gains
Maturing in one year or less	32,000	27,000
Maturing after one year through three years	84,000	56,000
Gross Unrealized Gains	116,000	83,000
Gross Unrealized Losses
Maturing in one year or less		(4,000)
Maturing after one year through three years	(23,000)	(4,000)
Gross Unrealized Losses	(23,000)	(8,000)
Fair Value
Maturing in one year or less	40,999,000	55,554,000
Maturing after one year through three years	43,309,000	18,286,000
Fair Value	84,308,000	73,840,000
Corporate debt securities
Amortized Cost
Maturing in one year or less	96,375,000	38,973,000
Maturing after one year through three years	36,651,000	20,761,000
Amortized cost	133,026,000	59,734,000
Gross Unrealized Gains
Maturing in one year or less	38,000	9,000
Maturing after one year through three years	12,000	12,000
Gross Unrealized Gains	50,000	21,000
Gross Unrealized Losses
Maturing in one year or less	(29,000)	(9,000)
Maturing after one year through three years	(31,000)	(5,000)
Gross Unrealized Losses	(60,000)	(14,000)
Fair Value
Maturing in one year or less	96,384,000	38,973,000
Maturing after one year through three years	36,632,000	20,768,000
Fair Value	$ 133,016,000	$ 59,741,000