Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 285.0	$ 205.8	$ 221.0	$ 144.2
Accounts receivable, net	2,725.1	2,671.2
Merchandise inventory	590.7	454.3
Miscellaneous receivables	353.4	316.4
Prepaid expenses and other	178.3	149.1
Total current assets	4,132.5	3,796.8
Operating lease right-of-use assets	132.1	0.0
Property and equipment, net	155.2	156.1
Goodwill	2,528.2	2,462.8
Other intangible assets, net	696.0	712.2
Other assets	35.5	39.8
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	7,679.5	7,167.7
Current liabilities:
Accounts payable-trade	1,745.4	1,577.1
Accounts payable-inventory financing	500.0	429.3
Current maturities of long-term debt	25.7	25.3
Contract liabilities	192.1	178.3
Accrued expenses and other current liabilities	751.8	593.1
Total current liabilities	3,215.0	2,803.1
Long-term liabilities:
Debt	3,257.7	3,183.3
Deferred income taxes	132.7	141.9
Operating lease liabilities	93.0	0.0
Other liabilities	53.8	64.2
Total long-term liabilities	3,537.2	3,389.4
Total stockholders' equity	927.3	975.2	$ 996.9	$ 985.6
Total stockholders' equity		975.2
Total Liabilities and Stockholders' Equity	7,679.5	7,167.7
Reportable Legal Entities
Current assets:
Operating lease right-of-use assets	0.0
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(8.0)	(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	(8.0)	(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	(239.3)	(161.2)
Investment in and advances to subsidiaries	(3,997.9)	(4,002.7)
Total Assets	(4,245.2)	(4,180.8)
Current liabilities:
Accounts payable-trade	(8.0)	(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	(8.0)	(16.9)
Long-term liabilities:
Debt	0.0	0.0
Deferred income taxes	(1.3)	(1.4)
Operating lease liabilities	0.0
Other liabilities	(238.0)	(159.8)
Total long-term liabilities	(239.3)	(161.2)
Total stockholders' equity	(3,997.9)
Total stockholders' equity		(4,002.7)
Total Liabilities and Stockholders' Equity	(4,245.2)	(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	1.3	1.4
Investment in and advances to subsidiaries	926.0	973.8
Total Assets	927.3	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	927.3
Total stockholders' equity		975.2
Total Liabilities and Stockholders' Equity	927.3	975.2
Subsidiary Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	195.2	176.0
Accounts receivable, net	0.0	0.0
Merchandise inventory	0.0	0.0
Miscellaneous receivables	105.5	110.6
Prepaid expenses and other	18.5	17.1
Total current assets	319.2	303.7
Operating lease right-of-use assets	22.9
Property and equipment, net	76.5	82.3
Goodwill	751.8	751.8
Other intangible assets, net	245.3	252.5
Other assets	38.7	49.8
Investment in and advances to subsidiaries	3,071.9	3,028.9
Total Assets	4,526.3	4,469.0
Current liabilities:
Accounts payable-trade	24.3	39.2
Accounts payable-inventory financing	0.3	0.2
Current maturities of long-term debt	14.9	14.9
Contract liabilities	0.0	0.0
Accrued expenses and other current liabilities	253.2	217.6
Total current liabilities	292.7	271.9
Long-term liabilities:
Debt	3,194.4	3,121.3
Deferred income taxes	50.4	55.9
Operating lease liabilities	34.7
Other liabilities	28.1	46.1
Total long-term liabilities	3,307.6	3,223.3
Total stockholders' equity	926.0
Total stockholders' equity		973.8
Total Liabilities and Stockholders' Equity	4,526.3	4,469.0
Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0	0.0
Accounts receivable, net	2,283.4	2,331.2
Merchandise inventory	516.2	387.4
Miscellaneous receivables	230.3	187.7
Prepaid expenses and other	99.2	93.8
Total current assets	3,129.1	3,000.1
Operating lease right-of-use assets	77.2
Property and equipment, net	53.2	52.0
Goodwill	1,437.8	1,437.8
Other intangible assets, net	267.3	300.0
Other assets	8.0	9.6
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	4,972.6	4,799.5
Current liabilities:
Accounts payable-trade	1,452.2	1,387.9
Accounts payable-inventory financing	474.7	406.1
Current maturities of long-term debt	4.2	4.0
Contract liabilities	98.2	95.6
Accrued expenses and other current liabilities	397.0	306.7
Total current liabilities	2,426.3	2,200.3
Long-term liabilities:
Debt	4.3	4.3
Deferred income taxes	50.5	60.5
Operating lease liabilities	30.3
Other liabilities	2.1	5.7
Total long-term liabilities	87.2	70.5
Total stockholders' equity	2,459.1
Total stockholders' equity		2,528.7
Total Liabilities and Stockholders' Equity	4,972.6	4,799.5
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	97.8	46.7
Accounts receivable, net	441.7	340.0
Merchandise inventory	74.5	66.9
Miscellaneous receivables	17.6	18.1
Prepaid expenses and other	60.6	38.2
Total current assets	692.2	509.9
Operating lease right-of-use assets	32.0
Property and equipment, net	25.5	21.8
Goodwill	338.6	273.2
Other intangible assets, net	183.4	159.7
Other assets	226.8	140.2
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	1,498.5	1,104.8
Current liabilities:
Accounts payable-trade	276.9	166.9
Accounts payable-inventory financing	25.0	23.0
Current maturities of long-term debt	6.6	6.4
Contract liabilities	93.9	82.7
Accrued expenses and other current liabilities	101.6	68.8
Total current liabilities	504.0	347.8
Long-term liabilities:
Debt	59.0	57.7
Deferred income taxes	33.1	26.9
Operating lease liabilities	28.0
Other liabilities	261.6	172.2
Total long-term liabilities	381.7	256.8
Total stockholders' equity	612.8
Total stockholders' equity		500.2
Total Liabilities and Stockholders' Equity	1,498.5	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
Total stockholders' equity		0.0
Total Liabilities and Stockholders' Equity	$ 0.0	$ 0.0