Recent Accounting Pronouncements - Adoption of ASU, Expected Impact (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement Related Disclosures [Abstract]
Net sales	$ 3,838.6	$ 4,033.9	$ 3,994.4	$ 3,324.7	$ 3,492.4	$ 3,708.2	$ 3,664.6	$ 3,116.7	$ 15,191.5	$ 13,981.9	$ 12,988.7
Gross profit	614.4	642.0	641.1	552.6	577.9	614.3	610.5	524.5	$ 2,449.9	$ 2,327.2	2,115.8
Gross profit margin									16.10%	16.60%
Income from operations	221.6	243.7	231.1	169.8	197.2	237.5	223.5	161.0	$ 866.1	$ 819.2	742.0
Income tax expense									(137.3)	(248.0)	(243.9)
Net income	$ 195.2	$ 129.2	$ 141.0	$ 57.6	$ 103.2	$ 125.9	$ 117.5	$ 77.8	$ 523.0	$ 424.4	$ 403.1
Net income per common share:
Earnings per share, basic (in dollars per share)	$ 1.28	$ 0.84	$ 0.90	$ 0.36	$ 0.64	$ 0.78	$ 0.71	$ 0.47	$ 3.37	$ 2.59	$ 2.37
Earnings per share, diluted (in dollars per share)	$ 1.26	$ 0.83	$ 0.89	$ 0.35	$ 0.63	$ 0.76	$ 0.70	$ 0.46	$ 3.31	$ 2.56	$ 2.35
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net	$ 2,320.5				$ 2,168.6				$ 2,320.5	$ 2,168.6
Merchandise inventory	449.5				452.0				449.5	452.0
Miscellaneous receivables	336.5				234.9				336.5	234.9
Prepaid expenses and other	127.4				118.9				127.4	118.9
Total current assets	3,378.1				3,238.1				3,378.1	3,238.1
Other assets	40.8				36.0				40.8	36.0
Total assets	6,956.6				6,948.4				6,956.6	6,948.4
Deferred revenue	194.0				172.6				194.0	172.6
Income tax payable	15.1				2.6				15.1	2.6
Other accrued expenses	180.2				147.2				180.2	147.2
Total current liabilities	2,514.6				2,280.7				2,514.6	2,280.7
Total liabilities	5,973.7				5,902.9				5,973.7	5,902.9
Total stockholders’ equity	982.9				1,045.5				982.9	1,045.5	$ 1,095.9	$ 936.5
Pro Forma
Income Statement Related Disclosures [Abstract]
Net sales									14,832.9	13,672.7
Gross profit									$ 2,450.2	$ 2,328.3
Gross profit margin									16.50%	17.00%
Income from operations									$ 866.5	$ 820.0
Income tax expense									(137.6)	(248.1)
Net income									$ 523.1	$ 425.1
Net income per common share:
Earnings per share, basic (in dollars per share)									$ 3.37	$ 2.60
Earnings per share, diluted (in dollars per share)									$ 3.31	$ 2.56
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net	2,329.3				2,168.9				$ 2,329.3	$ 2,168.9
Merchandise inventory	411.5				423.9				411.5	423.9
Miscellaneous receivables	343.0				237.5				343.0	237.5
Prepaid expenses and other	168.3				154.2				168.3	154.2
Total current assets	3,396.3				3,248.2				3,396.3	3,248.2
Other assets	32.7				35.9				32.7	35.9
Total assets	6,966.7				6,958.4				6,966.7	6,958.4
Deferred revenue	158.8				143.5				158.8	143.5
Income tax payable	16.2				3.3				16.2	3.3
Other accrued expenses	221.8				183.2				221.8	183.2
Total current liabilities	2,522.1				2,288.3				2,522.1	2,288.3
Total liabilities	5,981.1				5,910.5				5,981.1	5,910.5
Total stockholders’ equity	985.6				1,047.9				985.6	1,047.9
Difference between Revenue Guidance in Effect before and after Topic 606 | Pro Forma | Accounting Standards Update 2014-09
Income Statement Related Disclosures [Abstract]
Net sales									(358.6)	(309.2)
Gross profit									$ 0.3	$ 1.1
Gross profit 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)									$ 0.00	$ 0.01
Earnings per share, diluted (in dollars per share)									$ 0.00	$ 0.00
Balance Sheet Related Disclosures [Abstract]
Accounts receivable, net	8.8				0.3				$ 8.8	$ 0.3
Merchandise inventory	(38.0)				(28.1)				(38.0)	(28.1)
Miscellaneous receivables	6.5				2.6				6.5	2.6
Prepaid expenses and other	40.9				35.3				40.9	35.3
Total current assets	18.2				10.1				18.2	10.1
Other assets	(8.1)				(0.1)				(8.1)	(0.1)
Total assets	10.1				10.0				10.1	10.0
Deferred revenue	(35.2)				(29.1)				(35.2)	(29.1)
Income tax payable	1.1				0.7				1.1	0.7
Other accrued expenses	41.6				36.0				41.6	36.0
Total current liabilities	7.5				7.6				7.5	7.6
Total liabilities	7.5				7.6				7.5	7.6
Total stockholders’ equity	$ 2.7				$ 2.4				$ 2.7	$ 2.4