Selected Quarterly Financial Results (Details) (USD $)	3 Months Ended																12 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Jun. 30, 2014		Mar. 31, 2014		Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012
IPO Related Expenses									$ 74,100,000
Revenue, Net	3,050,100,000		3,266,100,000		3,106,000,000		2,652,300,000		2,713,300,000		2,864,300,000		2,779,300,000		2,411,700,000		12,074,500,000	10,768,600,000		10,128,200,000
Gross Profit	491,900,000		507,300,000		496,900,000		425,200,000		448,300,000		458,400,000		451,600,000		402,000,000		1,921,300,000	1,760,300,000		1,669,600,000
Income (loss) from operations	164,300,000		184,700,000		188,200,000		135,800,000		142,000,000	[1]	92,900,000	[1]	153,600,000	[1]	120,100,000	[1]	673,000,000	508,600,000	[2]	510,600,000
Net income (loss)	51,800,000		55,600,000		86,600,000		50,900,000		60,000,000	[1]	(2,200,000)	[1]	46,700,000	[1]	28,300,000	[1]	244,900,000	132,800,000		119,000,000
Basic	$ 0.30	[3]	$ 0.33	[3]	$ 0.51	[3]	$ 0.30	[3]	$ 0.35	[1],[3]	$ (0.01)	[1],[3]	$ 0.32	[1],[3]	$ 0.19	[1],[3]	$ 1.44	$ 0.85		$ 0.82
Diluted	$ 0.30	[3]	$ 0.32	[3]	$ 0.50	[3]	$ 0.30	[3]	$ 0.35	[1],[3]	$ (0.01)	[1],[3]	$ 0.32	[1],[3]	$ 0.19	[1],[3]	$ 1.42	$ 0.84		$ 0.82
Corporate [Member]
Revenue, Net	1,691,000,000		1,622,700,000		1,656,200,000		1,505,600,000		1,553,000,000		1,465,800,000		1,537,400,000		1,403,900,000
Corporate [Member] | Medium/Large [Member]
Revenue, Net	1,440,300,000		1,374,800,000		1,395,400,000		1,274,800,000		1,322,300,000	[4]	1,241,300,000	[4]	1,308,500,000	[4]	1,180,500,000	[4]
Corporate [Member] | Small Business [Member]
Revenue, Net	250,700,000		247,900,000		260,800,000		230,800,000		230,700,000	[4]	224,500,000	[4]	228,900,000	[4]	223,400,000	[4]
Public [Member]
Revenue, Net	1,169,100,000		1,468,800,000		1,271,600,000		969,900,000		990,500,000		1,244,600,000		1,082,600,000		846,800,000
Public [Member] | Government [Member]
Revenue, Net	440,800,000		441,300,000		313,100,000		254,200,000		327,300,000		375,300,000		295,700,000		252,300,000
Public [Member] | Education [Member]
Revenue, Net	342,600,000		632,800,000		527,000,000		321,600,000		282,800,000		513,400,000		420,600,000		232,200,000
Public [Member] | Healthcare [Member]
Revenue, Net	385,700,000		394,700,000		431,500,000		394,100,000		380,400,000		355,900,000		366,300,000		362,300,000
Other [Member]
Revenue, Net	190,000,000		174,600,000		178,200,000		176,800,000		169,800,000		153,900,000		159,300,000		161,000,000		719,600,000	644,000,000		592,400,000
Income (loss) from operations																	$ 32,900,000	$ 27,200,000	[2]	$ 18,600,000

[1] (3)The third quarter of 2013 included pre-tax IPO-related charges of $74.1 million. See Note 9 for additional discussion of the IPO.

[2] (1) Includes $75.0 million of IPO- and secondary-offering related expenses, as follows: Corporate $26.4 million; Public $14.4 million; Other $3.6 million; and Headquarters $30.6 million.

[3] (1) Basic and diluted net income (loss) per share are computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share information may not equal annual basic and diluted net income (loss) per share.

[4] (2)Net sales for the corporate channels (medium/large and small business) have been restated for all periods presented above to conform with the new corporate hierarchy presented for first quarter of 2014.