Acquisitions and Dispositions (Acquired Finite-Lived Intangibles Assets) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 1,679
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets	1,561
Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets	118
JLT
Finite-Lived Intangible Assets [Line Items]
Intangible assets	1,545
JLT | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 1,435
Weighted Average Amortization Period	13 years
JLT | Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 110
Weighted Average Amortization Period	4 years
Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 134
Other | Customer relationships
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 126
Weighted Average Amortization Period	12 years
Other | Other
Finite-Lived Intangible Assets [Line Items]
Intangible assets	$ 8
Weighted Average Amortization Period	4 years