Consolidated Statements of Operations (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Statement [Abstract]
Net sales	$ 2,608.6	$ 2,747.4	$ 2,623.2	$ 2,300.9	$ 2,491.3	$ 2,620.7	$ 2,509.9	$ 2,145.7	$ 10,280.1		$ 9,767.6		$ 8,892.4
Cost of goods sold	2,454.0	2,596.0	2,479.9	2,176.5	2,348.0	2,479.9	2,372.9	2,029.7	9,706.4		9,230.5		8,415.6
Gross profit	154.6	151.4	143.3	124.4	143.3	140.8	137.0	116.0	573.7		537.1		476.8
Warehousing and distribution expenses	81.5	83.5	78.1	75.3	77.2	79.4	72.8	67.7	318.4		297.1		262.7
Selling, general and administrative expenses	49.5	47.0	44.0	43.9	41.6	41.3	42.9	42.5	184.4		168.3		153.7
Amortization of intangible assets	0.6	0.7	0.7	0.6	0.7	0.6	0.7	0.7	2.6		2.7		3.0
Total operating expenses	131.6	131.2	122.8	119.8	119.5	121.3	116.4	110.9	505.4		468.1		419.4
Income from operations	23.0	20.2	20.5	4.6	23.8	19.5	20.6	5.1	68.3		69.0		57.4
Interest expense	(0.6)	(0.5)	(0.6)	(0.7)	(0.6)	(0.6)	(0.8)	(0.7)	(2.4)		(2.7)		(2.2)
Interest income	0.2	0.1	0.2	0.1	0.1	0.1	0.1	0.1	0.6		0.5		0.4
Foreign currency transaction losses, net	(0.1)	0.2	(0.2)	0	(0.1)	(0.1)	(0.1)	(0.4)	(0.1)		(0.8)		(0.2)
Income before income taxes	22.5	20.0	19.9	4.0	23.2	18.9	19.8	4.1	66.4		66.0		55.4
Provision for income taxes	(7.9)	(6.3)	(7.9)	(1.6)	(8.2)	(6.6)	(8.1)	(1.5)	(23.7)		(24.4)		(21.5)
Net income	$ 14.6	$ 13.7	$ 12.0	$ 2.4	$ 15.0	$ 12.3	$ 11.7	$ 2.6	$ 42.7	[1]	$ 41.6	[1]	$ 33.9	[1]
Basic net income per common share (in dollars per share)	$ 0.63	$ 0.59	$ 0.52	$ 0.11	$ 0.65	$ 0.54	$ 0.51	$ 0.11	$ 1.85		$ 1.81		$ 1.48
Diluted net income per common share (in dollars per share)	$ 0.62	$ 0.59	$ 0.52	$ 0.10	$ 0.65	$ 0.53	$ 0.51	$ 0.11	$ 1.83		$ 1.79		$ 1.46
Basic weighted-average shares (in shares)	23.1	23.1	23.1	23.0	23.0	23.0	23.0	23.0	23.1		23.0		23.0
Diluted weighted-average shares (in shares)	23.4	23.3	23.2	23.2	23.2	23.2	23.2	23.2	23.3		23.2		23.2

[1] Amounts have been rounded for presentation purposes and might differ from unrounded results.