Recent Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Statement Related Disclosures [Abstract]
Net sales	$ 3,606.4	$ 3,256.0
Gross profit	603.9	$ 553.5
Gross profit margin		17.00%
Income from operations	204.1	$ 170.7
Income tax (expense) benefit	(38.7)	(16.3)
Net income	$ 127.0	$ 58.2
Net income per common share:
Basic (in dollars per share)	$ 0.83	$ 0.36
Diluted (in dollars per share)	$ 0.82	$ 0.36
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $6.2 for both periods	$ 2,274.0		$ 2,329.3
Merchandise inventory	485.8		411.5
Miscellaneous receivables	310.2		343.0
Prepaid expenses and other	188.9		168.3
Total current assets	3,479.9		3,396.3
Other assets	47.1		32.7
Total assets	7,027.9		6,966.7
Contract liabilities	193.6		158.8
Income taxes	56.7		16.2
Other	204.9		221.8
Total current liabilities	2,582.5		2,522.1
Total liabilities			5,981.1
Total stockholders’ equity	$ 996.9		982.9
Total stockholders’ equity			985.6
As Reported
Income Statement Related Disclosures [Abstract]
Net sales		$ 3,324.7
Gross profit		$ 552.6
Gross profit margin		16.60%
Income from operations		$ 169.8
Income tax (expense) benefit		(16.0)
Net income		$ 57.6
Net income per common share:
Basic (in dollars per share)		$ 0.36
Diluted (in dollars per share)		$ 0.35
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $6.2 for both periods			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		$ (68.7)
Gross profit		$ 0.9
Gross profit margin		0.40%
Income from operations		$ 0.9
Income tax (expense) benefit		(0.3)
Net income		$ 0.6
Net income per common share:
Basic (in dollars per share)		$ 0.00
Diluted (in dollars per share)		$ 0.01
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $6.2 for both periods			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