Fair Value of Financial Instruments - Fair Value Relating to Financial Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset-based credit facility	$ 140,587		$ 25,660
Amount outstanding under letters of credit	1,303
Convertible senior notes, unamortized discount	4,483		4,665
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	108,017	[1]	107,835	[1]
Asset-based credit facility	140,587		25,660
Amount outstanding under letters of credit	1,303		1,297	[2]
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible notes	160,031	[1]	138,668	[1]
Asset-based credit facility	140,587		25,660
Amount outstanding under letters of credit	$ 1,303		$ 1,297	[2]

[1] The carrying value of the convertible notes at March 31, 2014 and December 31, 2013, is net of the unamortized discount of $4,483,000 and $4,665,000, 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] mounts outstanding under the Company's standby letters of credit approximates the fair value as they represent the Company’s contingent obligation to perform in accordance with the underlying contracts. The fair value of this contingent obligation is categorized within Level 2 of the fair value hierarchy.