Segment Information (Financial Data By Reportable Segment) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Operating revenue	$ 78,404.8	$ 73,021.7	$ 70,191.4
Operating gain (loss)	4,753.1	4,418.4	4,001.4
Depreciation and amortization of property and equipment	515.6	474.3	457.1
Commercial and Specialty Business [Member]
Segment Reporting Information [Line Items]
Operating revenue	37,570.8	39,199.6	39,404.2
Operating gain (loss)	2,854.0	3,260.9	3,176.4
Depreciation and amortization of property and equipment	0.0	0.0	0.0
Government Business [Member]
Segment Reporting Information [Line Items]
Operating revenue	40,813.0	33,796.4	30,752.6
Operating gain (loss)	1,978.5	1,191.9	844.0
Depreciation and amortization of property and equipment	0.0	0.0	0.0
Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	21.0	25.7	34.6
Operating gain (loss)	(79.4)	(34.4)	(19.0)
Depreciation and amortization of property and equipment	$ 515.6	$ 474.3	$ 457.1