Condensed Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement [Abstract]
Net sales	$ 2,623.2	$ 2,509.9	$ 4,924.1	$ 4,655.6
Cost of goods sold	2,479.9	2,372.9	4,656.4	4,402.6
Gross profit	143.3	137.0	267.7	253.0
Warehousing and distribution expenses	78.1	72.8	153.4	140.5
Selling, general and administrative expenses	44.0	42.9	87.9	85.4
Amortization of intangible assets	0.7	0.7	1.3	1.4
Total operating expenses	122.8	116.4	242.6	227.3
Income from operations	20.5	20.6	25.1	25.7
Interest expense	(0.6)	(0.8)	(1.3)	(1.5)
Interest income	0.2	0.1	0.3	0.2
Foreign currency transaction loss, net	(0.2)	(0.1)	(0.2)	(0.5)
Income before income taxes	19.9	19.8	23.9	23.9
Provision for income taxes	(7.9)	(8.1)	(9.5)	(9.6)
Net income	$ 12.0	$ 11.7	$ 14.4	$ 14.3
Basic net income per common share (in dollars per share)	$ 0.52	$ 0.51	$ 0.63	$ 0.62
Diluted net income per common share (in dollars per share)	$ 0.52	$ 0.51	$ 0.62	$ 0.62
Basic weighted-average shares	23.1	23.0	23.1	23.0
Diluted weighted-average shares	23.2	23.2	23.2	23.2
Dividends declared and paid per common share (in dollars per share)	$ 0.11	$ 0.10	$ 0.22	$ 0.10