Acquisitions and Dispositions (Acquired Finite-Lived Intangible Assets) (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 1,805
Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1,702
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	103
Jardine Lloyd Thompson Group plc
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1,662
Jardine Lloyd Thompson Group plc | Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 1,566
Finite-lived intangible assets, remaining amortization period	13 years
Jardine Lloyd Thompson Group plc | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 96
Finite-lived intangible assets, remaining amortization period	5 years
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 143
Other | Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 136
Finite-lived intangible assets, remaining amortization period	12 years
Other | Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 7
Finite-lived intangible assets, remaining amortization period	3 years