Supplemental Guarantor Information - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 205.8	$ 144.2	$ 263.7	$ 37.6
Accounts receivable, net	2,671.2	2,329.3	2,168.9
Merchandise inventory	454.3	411.5	423.9
Miscellaneous receivables	316.4	343.0	237.5
Prepaid expenses and other	149.1	168.3	154.2
Total current assets	3,796.8	3,396.3	3,248.2
Property and equipment, net	156.1	161.1
Goodwill	2,462.8	2,479.6	2,455.0
Other intangible assets, net	712.2	897.0
Other assets	39.8	32.7	35.9
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	7,167.7	6,966.7	6,958.4
Current liabilities:
Accounts payable-trade	1,577.1	1,317.7
Accounts payable-inventory financing	429.3	498.0
Current maturities of long-term debt	25.3	25.5
Contract liabilities	178.3	158.8	143.5
Accrued expenses and other current liabilities	593.1	522.1
Total current liabilities	2,803.1	2,522.1	2,288.3
Long-term liabilities:
Debt	3,183.3	3,210.0
Deferred income taxes	141.9	196.3
Other liabilities	64.2	52.7
Total long-term liabilities	3,389.4	3,459.0
Total stockholders' equity	975.2	985.6	1,047.9	1,097.8
Total Liabilities and Stockholders' Equity	7,167.7	6,966.7
Reportable Legal Entities | Parent Guarantor
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	973.8	983.9
Total Assets	975.2	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	975.2	985.6
Total Liabilities and Stockholders' Equity	975.2	985.6
Reportable Legal Entities | Subsidiary Issuer
Current assets:
Cash and cash equivalents	176.0	113.7	222.7	45.1
Accounts receivable, net	0.0	0.0
Merchandise inventory	0.0	0.0
Miscellaneous receivables	110.6	103.9
Prepaid expenses and other	17.1	18.0
Total current assets	303.7	235.6
Property and equipment, net	82.3	95.0
Goodwill	751.8	751.8
Other intangible assets, net	252.5	280.1
Other assets	49.8	30.7
Investment in and advances to subsidiaries	3,028.9	3,066.1
Total Assets	4,469.0	4,459.3
Current liabilities:
Accounts payable-trade	39.2	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	217.6	173.3
Total current liabilities	271.9	231.7
Long-term liabilities:
Debt	3,121.3	3,134.2
Deferred income taxes	55.9	66.5
Other liabilities	46.1	43.0
Total long-term liabilities	3,223.3	3,243.7
Total stockholders' equity	973.8	983.9
Total Liabilities and Stockholders' Equity	4,469.0	4,459.3
Reportable Legal Entities | Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	0.0	0.0	3.1	0.0
Accounts receivable, net	2,331.2	2,015.7
Merchandise inventory	387.4	354.6
Miscellaneous receivables	187.7	211.1
Prepaid expenses and other	93.8	100.4
Total current assets	3,000.1	2,681.8
Property and equipment, net	52.0	43.5
Goodwill	1,437.8	1,439.0
Other intangible assets, net	300.0	424.5
Other assets	9.6	209.3
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	4,799.5	4,798.1
Current liabilities:
Accounts payable-trade	1,387.9	1,112.1
Accounts payable-inventory financing	406.1	480.9
Current maturities of long-term debt	4.0	3.8
Contract liabilities	95.6	87.5
Accrued expenses and other current liabilities	306.7	262.0
Total current liabilities	2,200.3	1,946.3
Long-term liabilities:
Debt	4.3	8.3
Deferred income taxes	60.5	100.1
Other liabilities	5.7	4.7
Total long-term liabilities	70.5	113.1
Total stockholders' equity	2,528.7	2,738.7
Total Liabilities and Stockholders' Equity	4,799.5	4,798.1
Reportable Legal Entities | Non-Guarantor Subsidiaries
Current assets:
Cash and cash equivalents	46.7	32.4	37.9	31.9
Accounts receivable, net	340.0	313.6
Merchandise inventory	66.9	56.9
Miscellaneous receivables	18.1	28.0
Prepaid expenses and other	38.2	49.9
Total current assets	509.9	480.8
Property and equipment, net	21.8	22.6
Goodwill	273.2	288.8
Other intangible assets, net	159.7	192.4
Other assets	140.2	2.6
Investment in and advances to subsidiaries	0.0	0.0
Total Assets	1,104.8	987.2
Current liabilities:
Accounts payable-trade	166.9	165.0
Accounts payable-inventory financing	23.0	16.1
Current maturities of long-term debt	6.4	6.8
Contract liabilities	82.7	71.3
Accrued expenses and other current liabilities	68.8	86.8
Total current liabilities	347.8	346.0
Long-term liabilities:
Debt	57.7	67.5
Deferred income taxes	26.9	31.4
Other liabilities	172.2	214.9
Total long-term liabilities	256.8	313.8
Total stockholders' equity	500.2	327.4
Total Liabilities and Stockholders' Equity	1,104.8	987.2
Reportable Legal Entities | Affiliated Entity
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	0.0
Total Liabilities and Stockholders' Equity	0.0	0.0
Consolidating Adjustments
Current assets:
Cash and cash equivalents	(16.9)	(1.9)	$ 0.0	$ (39.4)
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	(16.9)	(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	(161.2)	(211.6)
Investment in and advances to subsidiaries	(4,002.7)	(4,050.0)
Total Assets	(4,180.8)	(4,263.5)
Current liabilities:
Accounts payable-trade	(16.9)	(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	(16.9)	(1.9)
Long-term liabilities:
Debt	0.0	0.0
Deferred income taxes	(1.4)	(1.7)
Other liabilities	(159.8)	(209.9)
Total long-term liabilities	(161.2)	(211.6)
Total stockholders' equity	(4,002.7)	(4,050.0)
Total Liabilities and Stockholders' Equity	$ (4,180.8)	$ (4,263.5)