Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Income Statement [Abstract]
Net sales	$ 3,836.8	$ 3,993.9	$ 3,687.4	$ 3,011.3	$ 2,815.1	$ 2,991.6	$ 2,810.4	$ 2,452.3	$ 14,529.4		$ 11,069.4		$ 10,280.1
Cost of goods sold	3,637.8	3,795.0	3,499.5	2,860.2	2,645.0	2,820.0	2,651.5	2,315.0	13,792.5		10,431.5		9,706.4
Gross profit	199.0	198.9	187.9	151.1	170.1	171.6	158.9	137.3	736.9		637.9		573.7
Warehousing and distribution expenses	116.2	117.4	106.0	91.6	91.7	92.8	88.6	79.5	431.2		352.6		318.4
Selling, general and administrative expenses	50.3	57.6	53.0	49.4	48.4	52.8	47.5	47.3	210.3		196.0		184.4
Amortization of intangible assets	1.5	1.7	1.2	0.9	0.8	0.6	0.6	0.6	5.3		2.6		2.6
Total operating expenses	168.0	176.7	160.2	141.9	140.9	146.2	136.7	127.4	646.8		551.2		505.4
Income from operations	31.0	22.2	27.7	9.2	29.2	25.4	22.2	9.9	90.1		86.7		68.3
Interest expense	(2.0)	(1.5)	(1.0)	(0.8)	(0.6)	(0.6)	(0.7)	(0.6)	(5.3)		(2.5)		(2.4)
Interest income	0.1	0.0	0.0	0.1	0.1	0.1	0.1	0.2	0.2		0.5		0.6
Foreign currency transaction gains (losses), net	0.6	(0.5)	(0.3)	0.7	(0.5)	(0.7)	(0.2)	(0.4)	0.5		(1.8)		(0.1)
Income before income taxes	29.7	20.2	26.4	9.2	28.2	24.2	21.4	9.1	85.5		82.9		66.4
Provision for income taxes (Note 10)	(11.0)	(6.7)	(10.1)	(3.5)	(10.5)	(9.1)	(8.2)	(3.6)	(31.3)		(31.4)		(23.7)
Net income	$ 18.7	$ 13.5	$ 16.3	$ 5.7	$ 17.7	$ 15.1	$ 13.2	$ 5.5	$ 54.2	[1]	$ 51.5	[1]	$ 42.7	[1]
Basic net income per common share (in dollars per share)	$ 0.41	$ 0.29	$ 0.35	$ 0.12	$ 0.39	$ 0.33	$ 0.29	$ 0.12	$ 1.17		$ 1.12		$ 0.93
Diluted net income per common share (in dollars per share)	$ 0.41	$ 0.29	$ 0.35	$ 0.12	$ 0.38	$ 0.33	$ 0.29	$ 0.12	$ 1.17		$ 1.11		$ 0.92
Basic weighted-average shares (in shares)	46.2	46.3	46.3	46.4	46.4	46.2	46.2	46.4	46.3		46.2		46.2
Diluted weighted-average shares (in shares)	46.4	46.5	46.5	46.6	46.8	46.6	46.6	46.6	46.5		46.6		46.6

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