Condensed Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Statement [Abstract]
Net sales	$ 2,620.7	$ 2,314.9	$ 7,276.3	$ 6,702.9
Cost of goods sold	2,479.9	2,192.7	6,882.5	6,348.0
Gross profit	140.8	122.2	393.8	354.9
Warehousing and distribution expenses	79.4	68.4	219.9	198.0
Selling, general and administrative expenses	41.3	35.9	126.7	113.4
Amortization of intangible assets	0.6	0.7	2.0	2.4
Total operating expenses	121.3	105.0	348.6	313.8
Income from operations	19.5	17.2	45.2	41.1
Interest expense	(0.6)	(0.4)	(2.1)	(1.6)
Interest income	0.1	0.1	0.3	0.3
Foreign currency transaction losses, net	(0.1)	0	(0.6)	(0.1)
Income before income taxes	18.9	16.9	42.8	39.7
Provision for income taxes (Note 7)	(6.6)	(6.4)	(16.2)	(15.5)
Net income	$ 12.3	$ 10.5	$ 26.6	$ 24.2
Basic net income per common share (Note 9) (in dollars per share)	$ 1.07	$ 0.92	$ 2.32	$ 2.12
Diluted net income per common share (Note 9) (in dollars per share)	$ 1.06	$ 0.90	$ 2.29	$ 2.08
Basic weighted-average shares (Note 9) (in shares)	11.5	11.5	11.5	11.4
Diluted weighted-average shares (Note 9) (in shares)	11.6	11.7	11.6	11.6
Dividends declared and paid per common share (Note 11) (in dollars per share)	$ 0.19	$ 0.17	$ 0.38	$ 0.51