Segment Information - Percent of sales revenue by country (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
United States of America
Segment Reporting Information [Line Items]
Percentage of sales revenue	97.60%	97.30%	97.70%	97.70%
Mexico
Segment Reporting Information [Line Items]
Percentage of sales revenue	1.20%	1.00%	1.20%	0.90%
Canada
Segment Reporting Information [Line Items]
Percentage of sales revenue	1.10%	1.50%	1.00%	1.30%
All Other Countries (none greater than 0.07%)
Segment Reporting Information [Line Items]
Percentage of sales revenue	0.10%	0.20%	0.10%	0.10%