Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized Cost, rolling maturity
Amortized cost	$ 54,242	$ 41,348
Amortized Cost, fiscal maturity
Amortized cost	54,242	41,348
Gross Unrealized Gains, rolling maturity
Gross Unrealized Gains	194	147
Gross Unrealized Gains, fiscal maturity
Gross Unrealized Gains	194	147
Gross Unrealized Losses, rolling maturity
Gross Unrealized Losses	8	82
Gross Unrealized Losses, fiscal maturity
Gross Unrealized Losses	8	82
Fair Value, rolling maturity
Fair Value	54,428	41,413
Fair Value, fiscal maturity
Fair Value	54,428	41,413
U.S. government and municipal obligations
Amortized Cost, rolling maturity
Maturing in one year or less	12,338
Maturing after one year through three years	17,297
Amortized cost	29,635	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	29,635	30,801
Gross Unrealized Gains, rolling maturity
Maturing in one year or less	28
Maturing after one year through three years	114
Gross Unrealized Gains	142	142
Gross Unrealized Gains, fiscal maturity
Maturing in one year or less		20
Maturing after one year through three years		122
Gross Unrealized Gains	142	142
Gross Unrealized Losses, rolling maturity
Maturing in one year or less	2
Maturing after one year through three years	1
Gross Unrealized Losses	3	6
Gross Unrealized Losses, fiscal maturity
Maturing after one year through three years		6
Gross Unrealized Losses	3	6
Fair Value, rolling maturity
Maturing in one year or less	12,364
Maturing after one year through three years	17,410
Fair Value	29,774	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	29,774	30,937
Corporate debt securities
Amortized Cost, rolling maturity
Maturing in one year or less	17,817
Maturing after one year through three years	6,790
Amortized cost	24,607	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	24,607	10,547
Gross Unrealized Gains, rolling maturity
Maturing in one year or less	39
Maturing after one year through three years	13
Gross Unrealized Gains	52	5
Gross Unrealized Gains, fiscal maturity
Maturing in one year or less		3
Maturing after one year through three years		2
Gross Unrealized Gains	52	5
Gross Unrealized Losses, rolling maturity
Maturing in one year or less	3
Maturing after one year through three years	2
Gross Unrealized Losses	5	76
Gross Unrealized Losses, fiscal maturity
Maturing in one year or less		4
Maturing after one year through three years		72
Gross Unrealized Losses	5	76
Fair Value, rolling maturity
Maturing in one year or less	17,853
Maturing after one year through three years	6,801
Fair Value	24,654	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	$ 24,654	$ 10,476