Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 255.1	$ 144.2	$ 97.9	$ 263.7
Accounts receivable, net	2,650.5	2,329.3
Merchandise inventory	481.1	411.5
Miscellaneous receivables	370.4	343.0
Prepaid expenses and other	192.4	168.3
Total current assets	3,949.5	3,396.3
Property and equipment, net	147.8	161.1
Goodwill	2,469.5	2,479.6
Other intangible assets, net	758.2	897.0
Other assets	50.4	32.7
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	7,375.4	6,966.7
Current liabilities:
Accounts payable-trade	1,609.9	1,317.7
Accounts payable-inventory financing	407.6	498.0
Current maturities of long-term debt	32.0	25.5
Contract liabilities	188.5	158.8
Accrued expenses and other current liabilities	565.9	522.1
Total current liabilities	2,803.9	2,522.1
Long-term liabilities:
Debt	3,187.4	3,210.0
Deferred income taxes	158.4	196.3
Other liabilities	65.0	52.7
Total long-term liabilities	3,410.8	3,459.0
Total stockholders' equity	1,160.7	982.9
Total stockholders' equity		985.6
Total Liabilities and Stockholders' Equity	7,375.4	6,966.7
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(15.3)	(1.9)	(11.7)	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	(15.3)	(1.9)
Property and equipment, net	0.0	0.0
Goodwill	0.0	0.0
Other intangible assets, net	0.0	0.0
Other assets	(169.8)	(211.6)
Investment in and advances to subsidiaries	(4,274.6)	(4,050.0)
Total Assets	(4,459.7)	(4,263.5)
Current liabilities:
Accounts payable-trade	(15.3)	(1.9)
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	(15.3)	(1.9)
Long-term liabilities:
Debt	0.0	0.0
Deferred income taxes	(1.4)	(1.7)
Other liabilities	(168.4)	(209.9)
Total long-term liabilities	(169.8)	(211.6)
Total stockholders' equity	(4,274.6)
Total stockholders' equity		(4,050.0)
Total Liabilities and Stockholders' Equity	(4,459.7)	(4,263.5)
Parent Guarantor | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0	0.0	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	1.4	1.7
Investment in and advances to subsidiaries	1,159.3	983.9
Total Assets	1,160.7	985.6
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
Other liabilities	0.0	0.0
Total long-term liabilities	0.0	0.0
Total stockholders' equity	1,160.7
Total stockholders' equity		985.6
Total Liabilities and Stockholders' Equity	1,160.7	985.6
Subsidiary Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	220.9	113.7	79.3	222.7
Accounts receivable, net	0.0	0.0
Merchandise inventory	0.0	0.0
Miscellaneous receivables	123.6	103.9
Prepaid expenses and other	17.4	18.0
Total current assets	361.9	235.6
Property and equipment, net	84.4	95.0
Goodwill	751.8	751.8
Other intangible assets, net	272.7	280.1
Other assets	64.9	30.7
Investment in and advances to subsidiaries	3,115.3	3,066.1
Total Assets	4,651.0	4,459.3
Current liabilities:
Accounts payable-trade	24.9	42.5
Accounts payable-inventory financing	0.2	1.0
Current maturities of long-term debt	14.9	14.9
Contract liabilities	0.0	0.0
Accrued expenses and other current liabilities	217.1	173.3
Total current liabilities	257.1	231.7
Long-term liabilities:
Debt	3,124.2	3,134.2
Deferred income taxes	63.4	66.5
Other liabilities	47.0	43.0
Total long-term liabilities	3,234.6	3,243.7
Total stockholders' equity	1,159.3
Total stockholders' equity		983.9
Total Liabilities and Stockholders' Equity	4,651.0	4,459.3
Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0	0.0	0.0	3.1
Accounts receivable, net	2,338.3	2,015.7
Merchandise inventory	419.7	354.6
Miscellaneous receivables	228.6	211.1
Prepaid expenses and other	133.3	100.4
Total current assets	3,119.9	2,681.8
Property and equipment, net	40.9	43.5
Goodwill	1,439.0	1,439.0
Other intangible assets, net	317.4	424.5
Other assets	151.8	209.3
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	5,069.0	4,798.1
Current liabilities:
Accounts payable-trade	1,431.2	1,112.1
Accounts payable-inventory financing	395.1	480.9
Current maturities of long-term debt	4.1	3.8
Contract liabilities	112.8	87.5
Accrued expenses and other current liabilities	283.3	262.0
Total current liabilities	2,226.5	1,946.3
Long-term liabilities:
Debt	4.3	8.3
Deferred income taxes	68.6	100.1
Other liabilities	5.7	4.7
Total long-term liabilities	78.6	113.1
Total stockholders' equity	2,763.9
Total stockholders' equity		2,738.7
Total Liabilities and Stockholders' Equity	5,069.0	4,798.1
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	49.5	32.4	30.3	37.9
Accounts receivable, net	312.2	313.6
Merchandise inventory	61.4	56.9
Miscellaneous receivables	18.2	28.0
Prepaid expenses and other	41.7	49.9
Total current assets	483.0	480.8
Property and equipment, net	22.5	22.6
Goodwill	278.7	288.8
Other intangible assets, net	168.1	192.4
Other assets	2.1	2.6
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	954.4	987.2
Current liabilities:
Accounts payable-trade	169.1	165.0
Accounts payable-inventory financing	12.3	16.1
Current maturities of long-term debt	13.0	6.8
Contract liabilities	75.7	71.3
Accrued expenses and other current liabilities	65.5	86.8
Total current liabilities	335.6	346.0
Long-term liabilities:
Debt	58.9	67.5
Deferred income taxes	27.8	31.4
Other liabilities	180.7	214.9
Total long-term liabilities	267.4	313.8
Total stockholders' equity	351.4
Total stockholders' equity		327.4
Total Liabilities and Stockholders' Equity	954.4	987.2
Co-Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	0.0	0.0	$ 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
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