Consolidated Statements of Operations - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Net sales	$ 4,252.1	$ 4,502.6	$ 4,263.9	$ 3,939.3	$ 4,154.8	$ 4,422.6	$ 4,339.0	$ 3,754.1	$ 16,957.9	$ 16,670.5	$ 16,395.3
Cost of goods sold	4,030.3	4,267.6	4,050.8	3,720.9	3,924.3	4,176.0	4,100.1	3,545.9	16,069.6	15,746.3	15,527.8
Gross profit	221.8	235.0	213.1	218.4	230.5	246.6	238.9	208.2	888.3	924.2	867.5
Warehousing and distribution expenses	133.4	140.4	125.5	142.4	140.2	148.6	143.2	134.2	541.7	566.2	540.6
Selling, general and administrative expenses	61.2	57.3	59.8	63.9	63.1	61.8	64.6	65.9	242.2	255.4	245.1
Amortization of intangible assets	2.5	2.5	2.4	2.3	2.3	2.3	2.7	2.7	9.7	10.0	10.0
Total operating expenses	197.1	200.2	187.7	208.6	205.6	212.7	210.5	202.8	793.6	831.6	795.7
Income from operations	24.7	34.8	25.4	9.8	24.9	33.9	28.4	5.4	94.7	92.6	71.8
Interest expense, net	(2.1)	(2.1)	(2.8)	(3.5)	(3.6)	(4.2)	(3.2)	(3.4)	(10.5)	(14.4)	(13.7)
Foreign currency transaction (losses) gains, net	(0.2)	(0.5)	0.0	(0.2)	(0.5)	0.9	(1.0)	(0.2)	(0.9)	(0.8)	1.8
Income before income taxes	22.4	32.2	22.6	6.1	20.8	30.6	24.2	1.8	83.3	77.4	59.9
Provision for income taxes (Note 10)	(3.4)	(9.2)	(5.7)	(1.8)	(4.6)	(8.1)	(6.5)	(0.5)	(20.1)	(19.7)	(14.4)
Net income	$ 19.0	$ 23.0	$ 16.9	$ 4.3	$ 16.2	$ 22.5	$ 17.7	$ 1.3	$ 63.2	$ 57.7	$ 45.5
Earnings per share (Note 12):
Basic (in dollars per share)	$ 0.42	$ 0.51	$ 0.38	$ 0.09	$ 0.35	$ 0.49	$ 0.39	$ 0.03	$ 1.40	$ 1.26	$ 0.99
Diluted (in dollars per share)	$ 0.42	$ 0.51	$ 0.38	$ 0.09	$ 0.35	$ 0.49	$ 0.38	$ 0.03	$ 1.39	$ 1.25	$ 0.99
Weighted-average shares outstanding (Note 12):
Basic (in shares)	44.9	45.1	45.1	45.3	45.7	45.8	45.9	45.9	45.1	45.7	46.0
Diluted (in shares)	45.4	45.4	45.3	45.4	46.0	46.1	46.1	46.0	45.4	46.0	46.1