Segment Information (Financial Data By Reportable Segment) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenue	$ 22,096.7	$ 21,125.2	$ 66,614.2	$ 62,709.1
Operating gain (loss)	982.0	1,078.1	3,794.4	4,148.7
Commercial and Specialty Business [Member]
Segment Reporting Information [Line Items]
Operating revenue	10,052.1	9,656.8	30,650.5	29,064.9
Operating gain (loss)	534.6	637.7	2,804.9	3,006.0
Government Business [Member]
Segment Reporting Information [Line Items]
Operating revenue	12,037.3	11,462.4	35,946.4	33,627.4
Operating gain (loss)	457.5	478.9	1,069.4	1,254.4
Other [Member]
Segment Reporting Information [Line Items]
Operating revenue	7.3	6.0	17.3	16.8
Operating gain (loss)	$ (10.1)	$ (38.5)	$ (79.9)	$ (111.7)