Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 166.9		$ 205.8
Accounts receivable, net	2,899.5		2,671.2
Merchandise inventory	603.6		454.3
Miscellaneous receivables	432.2		316.4
Prepaid expenses and other	193.1		149.1
Total current assets	4,295.3		3,796.8
Operating lease right-of-use assets	84.2
Property and equipment, net	171.1		156.1
Goodwill	2,515.9		2,462.8
Other intangible assets, net	616.1		712.2
Other assets	20.5		39.8
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	7,703.1		7,167.7
Current liabilities:
Accounts payable-trade	1,900.0		1,577.1
Accounts payable-inventory financing	411.7		429.3
Current maturities of long-term debt	25.4		25.3
Contract liabilities	212.1		178.3
Accrued expenses and other current liabilities	691.8		593.1
Total current liabilities	3,241.0		2,803.1
Long-term liabilities:
Debt	3,259.0		3,183.3
Deferred income taxes	111.3		141.9
Operating lease liabilities	81.7
Other liabilities	75.0		64.2
Total long-term liabilities	3,527.0		3,389.4
Total stockholders' equity	935.1	$ 936.1	975.2	$ 1,160.7	$ 1,068.9	$ 985.6
Total Liabilities and Stockholders' Equity	7,703.1		7,167.7
Reportable Legal Entities
Current assets:
Operating lease right-of-use assets	0.0
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(35.7)		(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	(35.7)		(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	(182.0)		(161.2)
Investment in and advances to subsidiaries	(4,123.7)		(4,002.7)
Total Assets	(4,341.4)		(4,180.8)
Current liabilities:
Accounts payable-trade	(35.7)		(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	(35.7)		(16.9)
Long-term liabilities:
Debt	0.0		0.0
Deferred income taxes	(1.1)		(1.4)
Operating lease liabilities	0.0
Other liabilities	(180.9)		(159.8)
Total long-term liabilities	(182.0)		(161.2)
Total stockholders' equity	(4,123.7)		(4,002.7)
Total Liabilities and Stockholders' Equity	(4,341.4)		(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	1.0		1.4
Investment in and advances to subsidiaries	934.1		973.8
Total Assets	935.1		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	935.1		975.2
Total Liabilities and Stockholders' Equity	935.1		975.2
Subsidiary Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	124.1		176.0
Accounts receivable, net	0.0		0.0
Merchandise inventory	0.0		0.0
Miscellaneous receivables	103.1		110.6
Prepaid expenses and other	22.2		17.1
Total current assets	249.4		303.7
Operating lease right-of-use assets	23.2
Property and equipment, net	75.8		82.3
Goodwill	751.8		751.8
Other intangible assets, net	243.1		252.5
Other assets	27.0		49.8
Investment in and advances to subsidiaries	3,189.6		3,028.9
Total Assets	4,559.9		4,469.0
Current liabilities:
Accounts payable-trade	26.2		39.2
Accounts payable-inventory financing	0.1		0.2
Current maturities of long-term debt	14.9		14.9
Contract liabilities	0.0		0.0
Accrued expenses and other current liabilities	256.0		217.6
Total current liabilities	297.2		271.9
Long-term liabilities:
Debt	3,209.1		3,121.3
Deferred income taxes	51.7		55.9
Operating lease liabilities	33.0
Other liabilities	34.8		46.1
Total long-term liabilities	3,328.6		3,223.3
Total stockholders' equity	934.1		973.8
Total Liabilities and Stockholders' Equity	4,559.9		4,469.0
Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0		0.0
Accounts receivable, net	2,539.1		2,331.2
Merchandise inventory	525.2		387.4
Miscellaneous receivables	300.6		187.7
Prepaid expenses and other	122.6		93.8
Total current assets	3,487.5		3,000.1
Operating lease right-of-use assets	33.1
Property and equipment, net	68.4		52.0
Goodwill	1,437.9		1,437.8
Other intangible assets, net	206.2		300.0
Other assets	2.4		9.6
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	5,235.5		4,799.5
Current liabilities:
Accounts payable-trade	1,701.9		1,387.9
Accounts payable-inventory financing	382.0		406.1
Current maturities of long-term debt	4.2		4.0
Contract liabilities	127.1		95.6
Accrued expenses and other current liabilities	352.0		306.7
Total current liabilities	2,567.2		2,200.3
Long-term liabilities:
Debt	0.0		4.3
Deferred income taxes	32.0		60.5
Operating lease liabilities	25.2
Other liabilities	0.9		5.7
Total long-term liabilities	58.1		70.5
Total stockholders' equity	2,610.2		2,528.7
Total Liabilities and Stockholders' Equity	5,235.5		4,799.5
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	78.5		46.7
Accounts receivable, net	360.4		340.0
Merchandise inventory	78.4		66.9
Miscellaneous receivables	28.5		18.1
Prepaid expenses and other	48.3		38.2
Total current assets	594.1		509.9
Operating lease right-of-use assets	27.9
Property and equipment, net	26.9		21.8
Goodwill	326.2		273.2
Other intangible assets, net	166.8		159.7
Other assets	172.1		140.2
Investment in and advances to subsidiaries	0.0		0.0
Total Assets	1,314.0		1,104.8
Current liabilities:
Accounts payable-trade	207.6		166.9
Accounts payable-inventory financing	29.6		23.0
Current maturities of long-term debt	6.3		6.4
Contract liabilities	85.0		82.7
Accrued expenses and other current liabilities	83.8		68.8
Total current liabilities	412.3		347.8
Long-term liabilities:
Debt	49.9		57.7
Deferred income taxes	28.7		26.9
Operating lease liabilities	23.5
Other liabilities	220.2		172.2
Total long-term liabilities	322.3		256.8
Total stockholders' equity	579.4		500.2
Total Liabilities and Stockholders' Equity	1,314.0		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