Marketable Securities (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Amortized Cost
Amortized cost	$ 211,409,000	$ 133,499,000
Gross Unrealized Gains
Gross Unrealized Gains	198,000	104,000
Gross Unrealized Losses
Gross Unrealized Losses	(35,000)	(22,000)
Fair Value
Fair Value	211,572,000	133,581,000
Marketable securities considered to be other-than-temporarily impaired	0
Accrued interest included in marketable securities	1,700,000	900,000
US Government and Municipal Obligations [Member]
Amortized Cost
Maturing in one year or less	46,138,000	55,531,000
Maturing after one year through three years	39,447,000	18,234,000
Amortized cost	85,585,000	73,765,000
Gross Unrealized Gains
Maturing in one year or less	59,000	27,000
Maturing after one year through three years	89,000	56,000
Gross Unrealized Gains	148,000	83,000
Gross Unrealized Losses
Maturing in one year or less		(4,000)
Maturing after one year through three years	(11,000)	(4,000)
Gross Unrealized Losses	(11,000)	(8,000)
Fair Value
Maturing in one year or less	46,197,000	55,554,000
Maturing after one year through three years	39,525,000	18,286,000
Fair Value	85,722,000	73,840,000
Corporate Debt Securities [Member]
Amortized Cost
Maturing in one year or less	96,350,000	38,973,000
Maturing after one year through three years	29,474,000	20,761,000
Amortized cost	125,824,000	59,734,000
Gross Unrealized Gains
Maturing in one year or less	39,000	9,000
Maturing after one year through three years	11,000	12,000
Gross Unrealized Gains	50,000	21,000
Gross Unrealized Losses
Maturing in one year or less	(11,000)	(9,000)
Maturing after one year through three years	(13,000)	(5,000)
Gross Unrealized Losses	(24,000)	(14,000)
Fair Value
Maturing in one year or less	96,378,000	38,973,000
Maturing after one year through three years	29,472,000	20,768,000
Fair Value	$ 125,850,000	$ 59,741,000