Fair Value of Financial Instruments - Fair Value Relating to Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible senior notes, unamortized discount	$ 4,844		$ 5,367
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	107,656	[1]	107,133	[1]
Standby letters of credit	1,278		3,265	[2]
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	126,000	[1]	118,406	[1]
Standby letters of credit	$ 1,278		$ 3,265	[2]

[1] The carrying value of the convertible notes at September 30, 2013 and December 31, 2012, is net of the unamortized discount of $4,844,000 and $5,367,000, respectively (see Note 3). The fair value of the convertible notes was determined based on secondary quoted market prices, and as such is classified as Level 2 in the fair value hierarchy.

[2] Amounts outstanding under standby letters of credit represent the Company’s contingent obligation to perform in accordance with the underlying contracts to which they pertain. The fair value of standby letters is classified as Level 1 as it approximates the carrying value due to the short term nature of these obligations.