Acquisitions and Dispositions (Acquired Finite-Lived Intangible Assets) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 1,756
Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1,647
Other
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	109
Jardine Lloyd Thompson Group plc
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	1,613
Jardine Lloyd Thompson Group plc | Client relationships
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 1,511
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	$ 102
Finite-lived intangible assets, remaining amortization period	4 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