Fair Value (Financial Assets measured at Fair Value on a Recurring Basis) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	$ 19,889	$ 41,937
Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	16,089	25,399
Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	3,800	16,538
Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
Money market funds
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	16,089	24,789
Money market funds | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	16,089	24,789
Money market funds | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
Money market funds | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
Commercial paper
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis		1,500
Commercial paper | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis		0
Commercial paper | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis		1,500
Commercial paper | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis		0
Corporate bonds
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	1,005	9,522
Corporate bonds | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
Corporate bonds | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	1,005	9,522
Corporate bonds | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
U.S. Treasury obligations
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	2,000	5,516
U.S. Treasury obligations | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
U.S. Treasury obligations | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	2,000	5,516
U.S. Treasury obligations | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	0	0
Common shares of CO2 Solutions
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	795	610
Common shares of CO2 Solutions | Level 1
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis		610
Common shares of CO2 Solutions | Level 2
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	795	0
Common shares of CO2 Solutions | Level 3
Summary of financial instruments measured at fair value on a recurring basis
Total financial assets measured at fair value on a recurring basis	$ 0	$ 0