Basis of Presentation and Significant Accounting Policies - Allowance for doubtful accounts (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Shipping, Handling and Transportation Costs	$ 35,471	$ 35,171	$ 36,585
Allowance for Doubtful Accounts Receivable [Roll Forward]
Balance, beginning of year	3,463	3,529	3,584
Add Provision charged to expense	465	484	1,420
Add Recoveries	139	173	90
Less Charges against allowance	(692)	(723)	(1,565)
Balance, end of year	$ 3,375	$ 3,463	$ 3,529