Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash and Cash Equivalents
Cash Equivalent included in cash and cash equivalent	$ 67,300,000	$ 113,900,000
Restricted Cash
Restricted cash securing letters of credit	8,147,000	7,647,000
Available-for-Sale Securities
Minimum original maturity period of entity's short-term investments in order to classify them as available for sale	3 months
Other-than-temporary impairments	0	0	0
Letters of credit
Restricted Cash
Contingent liability under unused letters of credit	8,100,000	7,600,000
Restricted cash securing letters of credit	$ 8,100,000	$ 7,600,000