Summary of Significant Accounting Policies Investments in Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 96,573		$ 130,971
Available-for-sale Securities, Gross Unrealized Gains	14		21
Available-for-sale Securities, Gross Unrealized Losses	(997)		(975)
Available-for-sale Securities, Fair Value Disclosure	95,590		130,017
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	96,573	[1]	130,971
Available-for-sale Securities, Gross Unrealized Gains	14	[1]	21
Available-for-sale Securities, Gross Unrealized Losses	(997)	[1]	(975)
Available-for-sale Securities, Fair Value Disclosure	$ 95,590	[1]	$ 130,017

[1] (1) Maturity dates ranging from 2014 to 2015.