Marketable Securities - (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 123,013	$ 113,838
Available-for-sale Securities, Fair Value Disclosure	123,045	113,871
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Debt Maturities, within One Year, Amortized Cost Basis	108,018	107,151
Available-for-sale Securities, Debt Maturities, within One Year, Fair Value	108,039	107,177
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	14,995	6,687
Available-for-sale Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	15,006	6,694
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis	0	0
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value	0	0
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0	0
US Government and Government Agencies and Authorities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	123,013	113,838
Available-for-sale Securities, Gross Unrealized Gains	38	37
Available-for-sale Securities, Gross Unrealized Losses	6	4
Available-for-sale Securities, Fair Value Disclosure	123,045	113,871
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	37,340	26,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	6	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	0	0
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	123,013	113,838
Available-for-sale Securities, Gross Unrealized Gains	38	37
Available-for-sale Securities, Gross Unrealized Losses	6	4
Available-for-sale Securities, Fair Value Disclosure	123,045	113,871
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	37,340	26,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Losses	6	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Losses	$ 0	$ 0