Supplemental Guarantor Information - Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	$ 602.3	$ 439.1
Cash flows used in investing activities:
Capital expenditures	(53.4)	(58.6)
Net cash used in investing activities	(53.4)	(58.6)
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	688.9	1,279.1
Repayments of borrowings under revolving credit facilities	(681.9)	(1,087.5)
Repayments of long-term debt	(18.2)	(11.2)
Proceeds from issuance of long-term debt	0.0	2,083.0
Payments to extinguish long-term debt	0.0	(2,121.3)
Payments of debt financing costs	(1.0)	(9.6)
Net change in accounts payable-inventory financing	(90.1)	(41.4)
Premium payments on interest rate cap agreements	(12.6)	0.0
Proceeds from stock option exercises	24.4	9.1
Proceeds from Coworker Stock Purchase Plan	8.4	7.6
Repurchases of common stock	(232.5)	(534.0)
Payment of incentive compensation plan withholding taxes	(27.1)	(42.0)
Dividends	(95.5)	(74.7)
Repayment of intercompany loan	0.0	0.0
Other	1.9	(4.7)
Distributions and advances from (to) affiliates	0.0	0.0
Net cash used in financing activities	(435.3)	(547.6)
Effect of exchange rate changes on cash and cash equivalents	(2.7)	1.3
Net increase (decrease) in cash and cash equivalents	110.9	(165.8)
Cash and cash equivalents—beginning of period	144.2	263.7	$ 263.7
Cash and cash equivalents—end of period	255.1	97.9	144.2
Consolidating Adjustments
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	(7.4)	(22.0)
Cash flows used in investing activities:
Capital expenditures	0.0	0.0
Net cash used in investing activities	0.0	0.0
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	0.0	0.0
Repayments of borrowings under revolving credit facilities	0.0	0.0
Repayments of long-term debt	0.0	0.0
Proceeds from issuance of long-term debt		0.0
Payments to extinguish long-term debt		0.0
Payments of debt financing costs	0.0	0.0
Net change in accounts payable-inventory financing	0.0	0.0
Premium payments on interest rate cap agreements	0.0
Proceeds from stock option exercises	0.0	0.0
Proceeds from Coworker Stock Purchase Plan	0.0	0.0
Repurchases of common stock	0.0	0.0
Payment of incentive compensation plan withholding taxes	0.0	0.0
Dividends	0.0	0.0
Repayment of intercompany loan	0.0	0.0
Other	0.0	0.0
Distributions and advances from (to) affiliates	(6.0)	10.3
Net cash used in financing activities	(6.0)	10.3
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	(13.4)	(11.7)
Cash and cash equivalents—beginning of period	(1.9)	0.0	0.0
Cash and cash equivalents—end of period	(15.3)	(11.7)	(1.9)
Parent Guarantor | Reportable Legal Entities
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	0.0	42.4
Cash flows used in investing activities:
Capital expenditures	0.0	0.0
Net cash used in investing activities	0.0	0.0
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	0.0	0.0
Repayments of borrowings under revolving credit facilities	0.0	0.0
Repayments of long-term debt	0.0	0.0
Proceeds from issuance of long-term debt		0.0
Payments to extinguish long-term debt		0.0
Payments of debt financing costs	0.0	0.0
Net change in accounts payable-inventory financing	0.0	0.0
Premium payments on interest rate cap agreements	0.0
Proceeds from stock option exercises	0.0	0.0
Proceeds from Coworker Stock Purchase Plan	0.0	0.0
Repurchases of common stock	(232.5)	(534.0)
Payment of incentive compensation plan withholding taxes	(27.1)	0.0
Dividends	(95.5)	(74.7)
Repayment of intercompany loan	0.0	0.0
Other	0.0	0.0
Distributions and advances from (to) affiliates	355.1	566.3
Net cash used in financing activities	0.0	(42.4)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents—beginning of period	0.0	0.0	0.0
Cash and cash equivalents—end of period	0.0	0.0	0.0
Subsidiary Issuer | Reportable Legal Entities
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	(78.9)	(17.7)
Cash flows used in investing activities:
Capital expenditures	(40.9)	(40.0)
Net cash used in investing activities	(40.9)	(40.0)
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	640.0	1,233.3
Repayments of borrowings under revolving credit facilities	(640.0)	(1,057.0)
Repayments of long-term debt	(11.2)	(11.2)
Proceeds from issuance of long-term debt		2,083.0
Payments to extinguish long-term debt		(2,121.3)
Payments of debt financing costs	(1.0)	(9.6)
Net change in accounts payable-inventory financing	(0.8)	(1.2)
Premium payments on interest rate cap agreements	(12.6)
Proceeds from stock option exercises	24.4	9.1
Proceeds from Coworker Stock Purchase Plan	8.4	7.6
Repurchases of common stock	0.0	0.0
Payment of incentive compensation plan withholding taxes	0.0	(16.0)
Dividends	0.0	0.0
Repayment of intercompany loan	0.0	0.0
Other	6.6	0.2
Distributions and advances from (to) affiliates	213.2	(202.6)
Net cash used in financing activities	227.0	(85.7)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	107.2	(143.4)
Cash and cash equivalents—beginning of period	113.7	222.7	222.7
Cash and cash equivalents—end of period	220.9	79.3	113.7
Guarantor Subsidiaries
Cash flows used in financing activities:
Payments of debt financing costs		0.0
Guarantor Subsidiaries | Reportable Legal Entities
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	608.4	422.4
Cash flows used in investing activities:
Capital expenditures	(3.5)	(4.8)
Net cash used in investing activities	(3.5)	(4.8)
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	0.0	0.0
Repayments of borrowings under revolving credit facilities	0.0	0.0
Repayments of long-term debt	0.0	0.0
Proceeds from issuance of long-term debt		0.0
Payments to extinguish long-term debt		0.0
Payments of debt financing costs	0.0
Net change in accounts payable-inventory financing	(85.8)	(52.9)
Premium payments on interest rate cap agreements	0.0
Proceeds from stock option exercises	0.0	0.0
Proceeds from Coworker Stock Purchase Plan	0.0	0.0
Repurchases of common stock	0.0	0.0
Payment of incentive compensation plan withholding taxes	0.0	(24.0)
Dividends	0.0	0.0
Repayment of intercompany loan	47.5	34.3
Other	(4.3)	(4.1)
Distributions and advances from (to) affiliates	(562.3)	(374.0)
Net cash used in financing activities	(604.9)	(420.7)
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	(3.1)
Cash and cash equivalents—beginning of period	0.0	3.1	3.1
Cash and cash equivalents—end of period	0.0	0.0	0.0
Non-Guarantor Subsidiaries | Reportable Legal Entities
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	80.2	14.0
Cash flows used in investing activities:
Capital expenditures	(9.0)	(13.8)
Net cash used in investing activities	(9.0)	(13.8)
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	48.9	45.8
Repayments of borrowings under revolving credit facilities	(41.9)	(30.5)
Repayments of long-term debt	(7.0)	0.0
Proceeds from issuance of long-term debt		0.0
Payments to extinguish long-term debt		0.0
Payments of debt financing costs	0.0	0.0
Net change in accounts payable-inventory financing	(3.5)	12.7
Premium payments on interest rate cap agreements	0.0
Proceeds from stock option exercises	0.0	0.0
Proceeds from Coworker Stock Purchase Plan	0.0	0.0
Repurchases of common stock	0.0	0.0
Payment of incentive compensation plan withholding taxes	0.0	(2.0)
Dividends	0.0	0.0
Repayment of intercompany loan	(47.5)	(34.3)
Other	(0.4)	(0.8)
Distributions and advances from (to) affiliates	0.0	0.0
Net cash used in financing activities	(51.4)	(9.1)
Effect of exchange rate changes on cash and cash equivalents	(2.7)	1.3
Net increase (decrease) in cash and cash equivalents	17.1	(7.6)
Cash and cash equivalents—beginning of period	32.4	37.9	37.9
Cash and cash equivalents—end of period	49.5	30.3	32.4
Co-Issuer | Reportable Legal Entities
Supplemental Guarantor Information [Line Items]
Net cash provided by operating activities	0.0	0.0
Cash flows used in investing activities:
Capital expenditures	0.0	0.0
Net cash used in investing activities	0.0	0.0
Cash flows used in financing activities:
Proceeds from borrowings under revolving credit facilities	0.0	0.0
Repayments of borrowings under revolving credit facilities	0.0	0.0
Repayments of long-term debt	0.0	0.0
Proceeds from issuance of long-term debt		0.0
Payments to extinguish long-term debt		0.0
Payments of debt financing costs	0.0	0.0
Net change in accounts payable-inventory financing	0.0	0.0
Premium payments on interest rate cap agreements	0.0
Proceeds from stock option exercises	0.0	0.0
Proceeds from Coworker Stock Purchase Plan	0.0	0.0
Repurchases of common stock	0.0	0.0
Payment of incentive compensation plan withholding taxes	0.0	0.0
Dividends	0.0	0.0
Repayment of intercompany loan	0.0	0.0
Other	0.0	0.0
Distributions and advances from (to) affiliates	0.0	0.0
Net cash used in financing activities	0.0	0.0
Effect of exchange rate changes on cash and cash equivalents	0.0	0.0
Net increase (decrease) in cash and cash equivalents	0.0	0.0
Cash and cash equivalents—beginning of period	0.0	0.0	0.0
Cash and cash equivalents—end of period	$ 0.0	$ 0.0	$ 0.0