Debt and Other Financing Arrangements - Schedule of Note Interest Expense (Details) - Convertible Debt - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 17, 2013
Debt Instrument [Line Items]
Contractual interest expense at 1.50% and 5.75% per annum	$ 4,897	$ 3,795	$ 3,795
Amortization of debt issuance costs	1,472	1,263	1,202
Accretion of debt discount	8,360	7,867	7,489
Total	$ 14,729	$ 12,925	$ 12,486
The 2018 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued				1.50%
The 2021 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued	5.75%