Recent Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Statement Related Disclosures [Abstract]
Net sales	$ 4,373.2	$ 3,933.2	$ 12,165.7	$ 11,080.8
Gross profit	713.6	$ 642.2	2,013.1	$ 1,836.5
Gross profit margin		16.30%		16.60%
Income from operations	274.8	$ 244.0	744.4	$ 645.6
Income tax (expense) benefit	(54.7)	(77.6)	(150.2)	(148.4)
Net income	$ 183.7	$ 129.3	$ 483.7	$ 328.3
Net income per common share:
Basic (in dollars per share)	$ 1.22	$ 0.84	$ 3.19	$ 2.10
Diluted (in dollars per share)	$ 1.20	$ 0.83	$ 3.14	$ 2.06
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $8.0 and $6.2, respectively	$ 2,650.5		$ 2,650.5		$ 2,329.3
Merchandise inventory	481.1		481.1		411.5
Miscellaneous receivables	370.4		370.4		343.0
Prepaid expenses and other	192.4		192.4		168.3
Total current assets	3,949.5		3,949.5		3,396.3
Other assets	50.4		50.4		32.7
Total assets	7,375.4		7,375.4		6,966.7
Contract liabilities	188.5		188.5		158.8
Income taxes	11.2		11.2		16.2
Other	209.2		209.2		221.8
Total current liabilities	2,803.9		2,803.9		2,522.1
Total liabilities					5,981.1
Total stockholders’ equity	$ 1,160.7		$ 1,160.7		982.9
Total stockholders’ equity					985.6
As Reported
Income Statement Related Disclosures [Abstract]
Net sales		$ 4,033.9		$ 11,353.0
Gross profit		$ 642.0		$ 1,835.6
Gross profit margin		15.90%		16.20%
Income from operations		$ 243.7		$ 644.6
Income tax (expense) benefit		(77.4)		(147.9)
Net income		$ 129.2		$ 327.8
Net income per common share:
Basic (in dollars per share)		$ 0.84		$ 2.10
Diluted (in dollars per share)		$ 0.83		$ 2.06
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $8.0 and $6.2, respectively					2,320.5
Merchandise inventory					449.5
Miscellaneous receivables					336.5
Prepaid expenses and other					127.4
Total current assets					3,378.1
Other assets					40.8
Total assets					6,956.6
Contract liabilities					194.0
Income taxes					15.1
Other					180.2
Total current liabilities					2,514.6
Total liabilities					5,973.7
Total stockholders’ equity					982.9
New Revenue Standard Adjustment | Accounting Standards Update 2014-09
Income Statement Related Disclosures [Abstract]
Net sales		$ (100.7)		$ (272.2)
Gross profit		$ 0.2		$ 0.9
Gross profit margin		0.40%		0.40%
Income from operations		$ 0.3		$ 1.0
Income tax (expense) benefit		(0.2)		(0.5)
Net income		$ 0.1		$ 0.5
Net income per common share:
Basic (in dollars per share)		$ 0		$ 0
Diluted (in dollars per share)		$ 0		$ 0
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $8.0 and $6.2, respectively					8.8
Merchandise inventory					(38.0)
Miscellaneous receivables					6.5
Prepaid expenses and other					40.9
Total current assets					18.2
Other assets					(8.1)
Total assets					10.1
Contract liabilities					(35.2)
Income taxes					1.1
Other					41.6
Total current liabilities					7.5
Total liabilities					7.5
Total stockholders’ equity					$ 2.7