Recent Accounting Pronouncements - Adoption of ASU Impact (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement Related Disclosures [Abstract]
Net sales	$ 4,074.8	$ 4,373.2	$ 4,186.1	$ 3,606.4	$ 3,752.1	$ 3,933.2	$ 3,891.7	$ 3,256.0	$ 16,240.5	$ 14,832.9	$ 13,672.7
Gross profit	693.8	713.6	695.6	603.9	613.7	642.2	640.8	553.5	2,706.9	$ 2,450.2	$ 2,328.3
Gross margin										16.50%	17.00%
Income from operations	242.9	274.8	265.5	204.1	221.1	244.0	230.8	170.7	987.3	$ 866.5	$ 820.0
Income tax expense									(197.5)	(137.6)	(248.1)
Net income	$ 159.3	$ 183.7	$ 173.0	$ 127.0	$ 194.8	$ 129.3	$ 140.9	$ 58.2	$ 643.0	$ 523.1	$ 425.1
Net income per common share:
Earnings per share, basic (in dollars per share)	$ 1.07	$ 1.22	$ 1.14	$ 0.83	$ 1.28	$ 0.84	$ 0.90	$ 0.36	$ 4.26	$ 3.37	$ 2.60
Earnings per share, diluted (in dollars per share)	$ 1.05	$ 1.20	$ 1.12	$ 0.82	$ 1.25	$ 0.83	$ 0.89	$ 0.36	$ 4.19	$ 3.31	$ 2.56
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net	$ 2,671.2				$ 2,329.3				$ 2,671.2	$ 2,329.3	$ 2,168.9
Merchandise inventory	454.3				411.5				454.3	411.5	423.9
Miscellaneous receivables	316.4				343.0				316.4	343.0	237.5
Prepaid expenses and other	149.1				168.3				149.1	168.3	154.2
Total current assets	3,796.8				3,396.3				3,796.8	3,396.3	3,248.2
Other assets	39.8				32.7				39.8	32.7	35.9
Total assets	7,167.7				6,966.7				7,167.7	6,966.7	6,958.4
Contract liabilities	178.3				158.8				178.3	158.8	143.5
Other accrued expenses					221.8					221.8	183.2
Income tax payable					16.2					16.2	3.3
Total current liabilities	2,803.1				2,522.1				2,803.1	2,522.1	2,288.3
Total liabilities					5,981.1					5,981.1	5,910.5
Total stockholders’ equity	$ 975.2				985.6				$ 975.2	985.6	1,047.9	$ 1,097.8
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					985.6					985.6	1,047.9
As Reported
Income Statement Related Disclosures [Abstract]
Net sales										15,191.5	13,981.9
Gross profit										$ 2,449.9	$ 2,327.2
Gross margin										16.10%	16.60%
Income from operations										$ 866.1	$ 819.2
Income tax expense										(137.3)	(248.0)
Net income										$ 523.0	$ 424.4
Net income per common share:
Earnings per share, basic (in dollars per share)										$ 3.37	$ 2.59
Earnings per share, diluted (in dollars per share)										$ 3.31	$ 2.56
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net					2,320.5					$ 2,320.5	$ 2,168.6
Merchandise inventory					449.5					449.5	452.0
Miscellaneous receivables					336.5					336.5	234.9
Prepaid expenses and other					127.4					127.4	118.9
Total current assets					3,378.1					3,378.1	3,238.1
Other assets					40.8					40.8	36.0
Total assets					6,956.6					6,956.6	6,948.4
Contract liabilities					194.0					194.0	172.6
Other accrued expenses					180.2					180.2	147.2
Income tax payable					15.1					15.1	2.6
Total current liabilities					2,514.6					2,514.6	2,280.7
Total liabilities					5,973.7					5,973.7	5,902.9
Total stockholders’ equity												$ 1,095.9
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					982.9					982.9	1,045.5
New Revenue Standard Adjustment | Accounting Standards Update 2014-09
Income Statement Related Disclosures [Abstract]
Net sales										(358.6)	(309.2)
Gross profit										$ 0.3	$ 1.1
Gross margin										0.40%	0.40%
Income from operations										$ 0.4	$ 0.8
Income tax expense										(0.3)	(0.1)
Net income										$ 0.1	$ 0.7
Net income per common share:
Earnings per share, basic (in dollars per share)										$ 0	$ 0.01
Earnings per share, diluted (in dollars per share)										$ 0	$ 0
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net					8.8					$ 8.8	$ 0.3
Merchandise inventory					(38.0)					(38.0)	(28.1)
Miscellaneous receivables					6.5					6.5	2.6
Prepaid expenses and other					40.9					40.9	35.3
Total current assets					18.2					18.2	10.1
Other assets					(8.1)					(8.1)	(0.1)
Total assets					10.1					10.1	10.0
Contract liabilities					(35.2)					(35.2)	(29.1)
Other accrued expenses					41.6					41.6	36.0
Income tax payable					1.1					1.1	0.7
Total current liabilities					7.5					7.5	7.6
Total liabilities					7.5					7.5	7.6
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					$ 2.7					$ 2.7	$ 2.4