Segment Information (Financial Data By Reportable Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenue	$ 84,194.0	$ 78,404.8	$ 73,021.7
Operating gain (loss)	4,801.7	4,753.1	4,418.4
Depreciation and amortization of property and equipment	576.0	515.6	474.3
Commercial and Specialty Business Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue	38,692.1	37,570.8	39,199.6
Operating gain (loss)	3,195.2	2,854.0	3,260.9
Depreciation and amortization of property and equipment	0.0	0.0	0.0
Government Business Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue	45,477.7	40,813.0	33,796.4
Operating gain (loss)	1,784.3	1,978.5	1,191.9
Depreciation and amortization of property and equipment	0.0	0.0	0.0
Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	24.2	21.0	25.7
Operating gain (loss)	(177.8)	(79.4)	(34.4)
Depreciation and amortization of property and equipment	$ 576.0	$ 515.6	$ 474.3