Basis of Presentation and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Supplemental Schedule of Non-Cash Investing and Financing Activities

Consolidated Statements of Cash Flows - Supplemental Schedule of Non-Cash Investing and Financing Activities

	Year Ended December 31,
	2017		2016		2015
	(in thousands)
Accrued purchases of property, equipment, and software	$	2,195	$	2,728	$	851
Accretion of dividend equivalents	3,532		3,244		2,978
Vendor financed insurance premiums	1,450		2,082		—

Fair Value Measurements, Valuation Techniques

The three levels of the hierarchy and the related inputs are as follows:

Level	Inputs
1	Unadjusted quoted prices in active markets for identical assets and liabilities.
2	Unadjusted quoted prices in active markets for similar assets and liabilities;
	Unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active; or
	Inputs other than quoted prices that are observable for the asset or liability.
3	Unobservable inputs for the asset or liability.