Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 100.7	$ 144.2	$ 79.0	$ 263.7
Accounts receivable, net	2,612.2	2,329.3
Merchandise inventory	576.9	411.5
Miscellaneous receivables	373.8	343.0
Prepaid expenses and other	181.1	168.3
Total current assets	3,844.7	3,396.3
Property and equipment, net	150.3	161.1
Goodwill	2,472.5	2,479.6
Other intangible assets, net	802.3	897.0
Other assets	60.3	32.7
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	7,330.1	6,966.7
Current liabilities:
Accounts payable-trade	1,693.6	1,317.7
Accounts payable-inventory financing	387.6	498.0
Current maturities of long-term debt	38.6	25.5
Contract liabilities	169.9	158.8
Accrued expenses and other current liabilities	530.3	522.1
Total current liabilities	2,820.0	2,522.1
Long-term liabilities:
Debt	3,201.7	3,210.0
Deferred income taxes	172.9	196.3
Other liabilities	66.6	52.7
Total long-term liabilities	3,441.2	3,459.0
Total stockholders' equity	1,068.9	982.9
Total stockholders' equity		985.6
Total Liabilities and Stockholders' Equity	7,330.1	6,966.7
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(22.6)	(1.9)	(26.5)	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	(22.6)	(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	(171.2)	(211.6)
Investment in and advances to subsidiaries	(4,226.0)	(4,050.0)
Total Assets	(4,419.8)	(4,263.5)
Current liabilities:
Accounts payable-trade	(22.6)	(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	(22.6)	(1.9)
Long-term liabilities:
Debt	0.0	0.0
Deferred income taxes	(1.5)	(1.7)
Other liabilities	(169.7)	(209.9)
Total long-term liabilities	(171.2)	(211.6)
Total stockholders' equity	(4,226.0)
Total stockholders' equity		(4,050.0)
Total Liabilities and Stockholders' Equity	(4,419.8)	(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	1.5	1.7
Investment in and advances to subsidiaries	1,067.4	983.9
Total Assets	1,068.9	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	1,068.9
Total stockholders' equity		985.6
Total Liabilities and Stockholders' Equity	1,068.9	985.6
Subsidiary Issuer | Reportable Legal Entities
Current assets:
Cash and cash equivalents	70.9	113.7	67.0	222.7
Accounts receivable, net	0.0	0.0
Merchandise inventory	0.0	0.0
Miscellaneous receivables	114.5	103.9
Prepaid expenses and other	20.4	18.0
Total current assets	205.8	235.6
Property and equipment, net	85.9	95.0
Goodwill	751.8	751.8
Other intangible assets, net	273.6	280.1
Other assets	69.6	30.7
Investment in and advances to subsidiaries	3,158.6	3,066.1
Total Assets	4,545.3	4,459.3
Current liabilities:
Accounts payable-trade	31.9	42.5
Accounts payable-inventory financing	0.1	1.0
Current maturities of long-term debt	14.9	14.9
Contract liabilities	0.0	0.0
Accrued expenses and other current liabilities	186.6	173.3
Total current liabilities	233.5	231.7
Long-term liabilities:
Debt	3,127.2	3,134.2
Deferred income taxes	67.6	66.5
Other liabilities	49.6	43.0
Total long-term liabilities	3,244.4	3,243.7
Total stockholders' equity	1,067.4
Total stockholders' equity		983.9
Total Liabilities and Stockholders' Equity	4,545.3	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	2,263.8	2,015.7
Merchandise inventory	502.2	354.6
Miscellaneous receivables	244.1	211.1
Prepaid expenses and other	120.6	100.4
Total current assets	3,130.7	2,681.8
Property and equipment, net	40.7	43.5
Goodwill	1,439.0	1,439.0
Other intangible assets, net	353.1	424.5
Other assets	159.2	209.3
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	5,122.7	4,798.1
Current liabilities:
Accounts payable-trade	1,460.9	1,112.1
Accounts payable-inventory financing	380.1	480.9
Current maturities of long-term debt	3.8	3.8
Contract liabilities	87.4	87.5
Accrued expenses and other current liabilities	276.6	262.0
Total current liabilities	2,208.8	1,946.3
Long-term liabilities:
Debt	8.3	8.3
Deferred income taxes	78.3	100.1
Other liabilities	4.3	4.7
Total long-term liabilities	90.9	113.1
Total stockholders' equity	2,823.0
Total stockholders' equity		2,738.7
Total Liabilities and Stockholders' Equity	5,122.7	4,798.1
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	52.4	32.4	38.5	37.9
Accounts receivable, net	348.4	313.6
Merchandise inventory	74.7	56.9
Miscellaneous receivables	15.2	28.0
Prepaid expenses and other	40.1	49.9
Total current assets	530.8	480.8
Property and equipment, net	23.7	22.6
Goodwill	281.7	288.8
Other intangible assets, net	175.6	192.4
Other assets	1.2	2.6
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	1,013.0	987.2
Current liabilities:
Accounts payable-trade	223.4	165.0
Accounts payable-inventory financing	7.4	16.1
Current maturities of long-term debt	19.9	6.8
Contract liabilities	82.5	71.3
Accrued expenses and other current liabilities	67.1	86.8
Total current liabilities	400.3	346.0
Long-term liabilities:
Debt	66.2	67.5
Deferred income taxes	28.5	31.4
Other liabilities	182.4	214.9
Total long-term liabilities	277.1	313.8
Total stockholders' equity	335.6
Total stockholders' equity		327.4
Total Liabilities and Stockholders' Equity	1,013.0	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