Recent Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Statement Related Disclosures [Abstract]
Net sales	$ 4,186.1	$ 3,891.7	$ 7,792.5	$ 7,147.6
Gross profit	695.6	$ 640.8	1,299.5	$ 1,194.3
Gross profit margin		16.50%		16.70%
Income from operations	265.5	$ 230.8	469.6	$ 401.5
Income tax (expense) benefit	(56.8)	(54.4)	(95.5)	(70.8)
Net income	$ 173.0	$ 140.9	$ 300.0	$ 199.0
Net income per common share:
Basic (in dollars per share)	$ 1.14	$ 0.90	$ 1.98	$ 1.26
Diluted (in dollars per share)	$ 1.12	$ 0.89	$ 1.94	$ 1.24
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $7.1 and $6.2, respectively	$ 2,612.2		$ 2,612.2		$ 2,329.3
Merchandise inventory	576.9		576.9		411.5
Miscellaneous receivables	373.8		373.8		343.0
Prepaid expenses and other	181.1		181.1		168.3
Total current assets	3,844.7		3,844.7		3,396.3
Other assets	60.3		60.3		32.7
Total assets	7,330.1		7,330.1		6,966.7
Contract liabilities	169.9		169.9		158.8
Income taxes	22.1		22.1		16.2
Other	212.8		212.8		221.8
Total current liabilities	2,820.0		2,820.0		2,522.1
Total liabilities					5,981.1
Total stockholders’ equity	$ 1,068.9		$ 1,068.9		982.9
Total stockholders’ equity					985.6
As Reported
Income Statement Related Disclosures [Abstract]
Net sales		$ 3,994.4		$ 7,319.1
Gross profit		$ 641.1		$ 1,193.6
Gross profit margin		16.10%		16.30%
Income from operations		$ 231.1		$ 400.9
Income tax (expense) benefit		(54.5)		(70.5)
Net income		$ 141.0		$ 198.7
Net income per common share:
Basic (in dollars per share)		$ 0.90		$ 1.26
Diluted (in dollars per share)		$ 0.89		$ 1.23
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $7.1 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		$ (102.7)		$ (171.5)
Gross profit		$ (0.3)		$ 0.7
Gross profit margin		0.40%		0.40%
Income from operations		$ (0.3)		$ 0.6
Income tax (expense) benefit		0.1		(0.3)
Net income		$ (0.1)		$ 0.3
Net income per common share:
Basic (in dollars per share)		$ 0.00		$ 0.00
Diluted (in dollars per share)		$ 0.00		$ 0.01
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $7.1 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