Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized Cost, rolling maturity
Amortized cost	$ 59,975	$ 41,348
Amortized Cost, fiscal maturity
Amortized cost	59,975	41,348
Gross Unrealized Gains, rolling maturity
Gross Unrealized Gains	151	147
Gross Unrealized Gains, fiscal maturity
Gross Unrealized Gains	151	147
Gross Unrealized Losses, rolling maturity
Gross Unrealized Losses	17	82
Gross Unrealized Losses, fiscal maturity
Gross Unrealized Losses	17	82
Fair Value, rolling maturity
Fair Value	60,109	41,413
Fair Value, fiscal maturity
Fair Value	60,109	41,413
U.S. government and municipal obligations
Amortized Cost, rolling maturity
Maturing in one year or less	21,973
Maturing after one year through three years	15,543
Amortized cost	37,516	30,801
Amortized Cost, fiscal maturity
Maturing in one year or less		19,993
Maturing after one year through three years		10,808
Amortized cost	37,516	30,801
Gross Unrealized Gains, rolling maturity
Maturing in one year or less	27
Maturing after one year through three years	99
Gross Unrealized Gains	126	142
Gross Unrealized Gains, fiscal maturity
Maturing in one year or less		20
Maturing after one year through three years		122
Gross Unrealized Gains	126	142
Gross Unrealized Losses, rolling maturity
Maturing in one year or less	4
Maturing after one year through three years	1
Gross Unrealized Losses	5	6
Gross Unrealized Losses, fiscal maturity
Maturing after one year through three years		6
Gross Unrealized Losses	5	6
Fair Value, rolling maturity
Maturing in one year or less	21,996
Maturing after one year through three years	15,641
Fair Value	37,637	30,937
Fair Value, fiscal maturity
Maturing in one year or less		20,013
Maturing after one year through three years		10,924
Fair Value	37,637	30,937
Corporate debt securities
Amortized Cost, rolling maturity
Maturing in one year or less	19,156
Maturing after one year through three years	3,303
Amortized cost	22,459	10,547
Amortized Cost, fiscal maturity
Maturing in one year or less		5,817
Maturing after one year through three years		4,730
Amortized cost	22,459	10,547
Gross Unrealized Gains, rolling maturity
Maturing in one year or less	19
Maturing after one year through three years	6
Gross Unrealized Gains	25	5
Gross Unrealized Gains, fiscal maturity
Maturing in one year or less		3
Maturing after one year through three years		2
Gross Unrealized Gains	25	5
Gross Unrealized Losses, rolling maturity
Maturing in one year or less	4
Maturing after one year through three years	8
Gross Unrealized Losses	12	76
Gross Unrealized Losses, fiscal maturity
Maturing in one year or less		4
Maturing after one year through three years		72
Gross Unrealized Losses	12	76
Fair Value, rolling maturity
Maturing in one year or less	19,171
Maturing after one year through three years	3,301
Fair Value	22,472	10,476
Fair Value, fiscal maturity
Maturing in one year or less		5,816
Maturing after one year through three years		4,660
Fair Value	$ 22,472	$ 10,476