Segment Information Segment Reporting (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
Segment Reporting Information [Line Items]
IPO and Secondary-offering related expenses														$ 75,000,000
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
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
Depreciation and amortization expense													(207,900,000)	(208,200,000)		(210,200,000)
Segment Reporting Information, Additional Information [Abstract]
Number of Reportable Segments													2
Number of operating segments which do not meet reporting unit quantitative threshold													3
Net sales to the federal government, Percentage													7.00%	7.00%		10.00%
Net sales to customers outside of the US, Percent													4.00%	4.00%		4.00%
Long-Lived Assets in Foreign Countries, Percent	1.00%				1.00%								1.00%	1.00%
Equity Method Investment, Ownership Percentage	35.00%												35.00%
Public Segment: Government Agencies, Education and Healthcare [Member]
Segment Reporting Information [Line Items]
Revenue, Net													4,879,400,000	4,164,500,000		4,023,000,000
Income (loss) from operations													313,200,000	246,500,000	[2]	246,700,000
Depreciation and amortization expense													(43,800,000)	(44,000,000)		(44,000,000)
Corporate [Member]
Segment Reporting Information [Line Items]
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
Segment Reporting Information, Additional Information [Abstract]
IPO- and secondary-offering related expenses														26,400,000
Public [Member]
Segment Reporting Information [Line Items]
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
Segment Reporting Information, Additional Information [Abstract]
IPO- and secondary-offering related expenses														14,400,000
Other [Member]
Segment Reporting Information [Line Items]
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
Depreciation and amortization expense													(8,800,000)	(8,600,000)		(9,300,000)
Segment Reporting Information, Additional Information [Abstract]
IPO- and secondary-offering related expenses														3,600,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenue, Net													0	0		0
Income (loss) from operations													(112,900,000)	(128,400,000)	[2]	(103,700,000)
Depreciation and amortization expense													(59,000,000)	(58,300,000)		(59,300,000)
Segment Reporting Information, Additional Information [Abstract]
IPO- and secondary-offering related expenses														30,600,000
Corporate Segment: Private Sector Business [Member]
Segment Reporting Information [Line Items]
Revenue, Net													6,475,500,000	5,960,100,000		5,512,800,000
Income (loss) from operations													439,800,000	363,300,000	[2]	349,000,000
Depreciation and amortization expense													(96,300,000)	(97,300,000)		(97,600,000)
Federal [Member] | Public [Member]
Segment Reporting Information [Line Items]
Revenue, Net													$ 884,200,000	$ 764,400,000		$ 964,700,000

[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.