Leases - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Percentage of lease obligations used for office space		98.00%
Lessee, operating lease, lease not yet commenced, amount	$ 71	$ 71
Lessee, operating lease, lease not yet commenced, term (years)	12 months	12 months
Operating lease, right-of-use asset	$ 1,957	$ 1,957		$ 0
Operating lease, liability	2,298	2,298
Operating lease, impairment loss	$ 7	$ 7
Minimum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract (years)	10 years	10 years
Maximum
Lessee, Lease, Description [Line Items]
Lessee, operating lease, term of contract (years)	25 years	25 years
Jardine Lloyd Thompson Group plc
Lessee, Lease, Description [Line Items]
Operating lease, right-of-use asset			$ 400
Operating lease, liability			$ 400