Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Supplemental Guarantor Information [Line Items]
Document Period End Date	Jun. 30, 2019
Current assets:
Cash and cash equivalents	$ 194.5		$ 205.8
Accounts receivable, net	2,967.3		2,671.2
Merchandise inventory	652.2		454.3
Miscellaneous receivables	406.0		316.4
Prepaid expenses and other	205.3		149.1
Total current assets	4,425.3		3,796.8
Operating lease right-of-use assets	117.3
Property and equipment, net	163.4		156.1
Goodwill	2,524.7		2,462.8
Other intangible assets, net	660.5		712.2
Other assets	30.7		39.8
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	7,921.9		7,167.7
Current liabilities:
Accounts payable-trade	2,055.8		1,577.1
Accounts payable-inventory financing	465.7		429.3
Current maturities of long-term debt	25.5		25.3
Contract liabilities	200.8		178.3
Accrued expenses and other current liabilities	712.5		593.1
Total current liabilities	3,460.3		2,803.1
Long-term liabilities:
Debt	3,253.3		3,183.3
Deferred income taxes	123.1		141.9
Operating lease liabilities	86.6
Other liabilities	62.5		64.2
Total long-term liabilities	3,525.5		3,389.4
Total stockholders' equity	936.1	$ 927.3	975.2	$ 1,068.9	$ 996.9	$ 985.6
Total Liabilities and Stockholders' Equity	7,921.9		7,167.7
Reportable Legal Entities
Current assets:
Operating lease right-of-use assets	0.0
Consolidating Adjustments
Current assets:
Cash and cash equivalents	0.0		(16.9)
Accounts receivable, net	0.0		0.0
Merchandise inventory	0.0		0.0
Miscellaneous receivables	0.0		0.0
Prepaid expenses and other	0.0		0.0
Total current assets	0.0		(16.9)
Operating lease right-of-use assets	0.0
Property and equipment, net	0.0		0.0
Goodwill	0.0		0.0
Other intangible assets, net	0.0		0.0
Other assets	(199.6)		(161.2)
Investment in and advances to subsidiaries	(4,114.2)		(4,002.7)
Total Assets	(4,313.8)		(4,180.8)
Current liabilities:
Accounts payable-trade	0.0		(16.9)
Accounts payable-inventory financing	0.0		0.0
Current maturities of long-term debt	0.0		0.0
Contract liabilities	0.0		0.0
Accrued expenses and other current liabilities	0.0		0.0
Total current liabilities	0.0		(16.9)
Long-term liabilities:
Debt	0.0		0.0
Deferred income taxes	(1.2)		(1.4)
Operating lease liabilities	0.0
Other liabilities	(198.4)		(159.8)
Total long-term liabilities	(199.6)		(161.2)
Total stockholders' equity	(4,114.2)		(4,002.7)
Total Liabilities and Stockholders' Equity	(4,313.8)		(4,180.8)
Parent Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0		0.0
Accounts receivable, net	0.0		0.0
Merchandise inventory	0.0		0.0
Miscellaneous receivables	0.0		0.0
Prepaid expenses and other	0.0		0.0
Total current assets	0.0		0.0
Operating lease right-of-use assets	0.0
Property and equipment, net	0.0		0.0
Goodwill	0.0		0.0
Other intangible assets, net	0.0		0.0
Other assets	0.6		1.4
Investment in and advances to subsidiaries	935.5		973.8
Total Assets	936.1		975.2
Current liabilities:
Accounts payable-trade	0.0		0.0
Accounts payable-inventory financing	0.0		0.0
Current maturities of long-term debt	0.0		0.0
Contract liabilities	0.0		0.0
Accrued expenses and other current liabilities	0.0		0.0
Total current liabilities	0.0		0.0
Long-term liabilities:
Debt	0.0		0.0
Deferred income taxes	0.0		0.0
Operating lease liabilities	0.0
Other liabilities	0.0		0.0
Total long-term liabilities	0.0		0.0
Total stockholders' equity	936.1		975.2
Total Liabilities and Stockholders' Equity	936.1		975.2
Subsidiary Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	97.6		176.0
Accounts receivable, net	0.0		0.0
Merchandise inventory	0.0		0.0
Miscellaneous receivables	107.4		110.6
Prepaid expenses and other	25.9		17.1
Total current assets	230.9		303.7
Operating lease right-of-use assets	21.8
Property and equipment, net	81.8		82.3
Goodwill	751.8		751.8
Other intangible assets, net	246.4		252.5
Other assets	43.0		49.8
Investment in and advances to subsidiaries	3,178.7		3,028.9
Total Assets	4,554.4		4,469.0
Current liabilities:
Accounts payable-trade	38.4		39.2
Accounts payable-inventory financing	0.5		0.2
Current maturities of long-term debt	14.9		14.9
Contract liabilities	0.0		0.0
Accrued expenses and other current liabilities	251.8		217.6
Total current liabilities	305.6		271.9
Long-term liabilities:
Debt	3,191.4		3,121.3
Deferred income taxes	53.5		55.9
Operating lease liabilities	32.6
Other liabilities	35.8		46.1
Total long-term liabilities	3,313.3		3,223.3
Total stockholders' equity	935.5		973.8
Total Liabilities and Stockholders' Equity	4,554.4		4,469.0
Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	23.8		0.0
Accounts receivable, net	2,584.9		2,331.2
Merchandise inventory	577.9		387.4
Miscellaneous receivables	276.4		187.7
Prepaid expenses and other	124.2		93.8
Total current assets	3,587.2		3,000.1
Operating lease right-of-use assets	65.2
Property and equipment, net	54.7		52.0
Goodwill	1,437.8		1,437.8
Other intangible assets, net	237.2		300.0
Other assets	7.8		9.6
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	5,389.9		4,799.5
Current liabilities:
Accounts payable-trade	1,771.6		1,387.9
Accounts payable-inventory financing	451.2		406.1
Current maturities of long-term debt	4.1		4.0
Contract liabilities	100.5		95.6
Accrued expenses and other current liabilities	378.8		306.7
Total current liabilities	2,706.2		2,200.3
Long-term liabilities:
Debt	4.3		4.3
Deferred income taxes	40.9		60.5
Operating lease liabilities	27.8
Other liabilities	4.9		5.7
Total long-term liabilities	77.9		70.5
Total stockholders' equity	2,605.8		2,528.7
Total Liabilities and Stockholders' Equity	5,389.9		4,799.5
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	73.1		46.7
Accounts receivable, net	382.4		340.0
Merchandise inventory	74.3		66.9
Miscellaneous receivables	22.2		18.1
Prepaid expenses and other	55.2		38.2
Total current assets	607.2		509.9
Operating lease right-of-use assets	30.3
Property and equipment, net	26.9		21.8
Goodwill	335.1		273.2
Other intangible assets, net	176.9		159.7
Other assets	178.9		140.2
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	1,355.3		1,104.8
Current liabilities:
Accounts payable-trade	245.8		166.9
Accounts payable-inventory financing	14.0		23.0
Current maturities of long-term debt	6.5		6.4
Contract liabilities	100.3		82.7
Accrued expenses and other current liabilities	81.9		68.8
Total current liabilities	448.5		347.8
Long-term liabilities:
Debt	57.6		57.7
Deferred income taxes	29.9		26.9
Operating lease liabilities	26.2
Other liabilities	220.2		172.2
Total long-term liabilities	333.9		256.8
Total stockholders' equity	572.9		500.2
Total Liabilities and Stockholders' Equity	1,355.3		1,104.8
Affiliated Entity | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0		0.0
Accounts receivable, net	0.0		0.0
Merchandise inventory	0.0		0.0
Miscellaneous receivables	0.0		0.0
Prepaid expenses and other	0.0		0.0
Total current assets	0.0		0.0
Property and equipment, net	0.0		0.0
Goodwill	0.0		0.0
Other intangible assets, net	0.0		0.0
Other assets	0.0		0.0
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	0.0		0.0
Current liabilities:
Accounts payable-trade	0.0		0.0
Accounts payable-inventory financing	0.0		0.0
Current maturities of long-term debt	0.0		0.0
Contract liabilities	0.0		0.0
Accrued expenses and other current liabilities	0.0		0.0
Total current liabilities	0.0		0.0
Long-term liabilities:
Debt	0.0		0.0
Deferred income taxes	0.0		0.0
Operating lease liabilities	0.0
Other liabilities	0.0		0.0
Total long-term liabilities	0.0		0.0
Total stockholders' equity	0.0		0.0
Total Liabilities and Stockholders' Equity	$ 0.0		$ 0.0