Investments (Leased Real Estate Investments - Operating Leases) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	$ 4,372		$ 4,132
Office
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	2,124		1,999
Operating Lease, Lease Income	44	$ 45
Retail
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	1,090		1,006
Operating Lease, Lease Income	24	26
Apartment
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	250		253
Operating Lease, Lease Income	5	18
Industrial
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	275		223
Operating Lease, Lease Income	11	9
Land
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	473		489
Operating Lease, Lease Income	5	5
Hotel
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	93		94
Operating Lease, Lease Income	2	0
Other
Schedule of Operating Leases by Property Type [Line Items]
Leased real estate investments, Carrying Value	67		$ 68
Operating Lease, Lease Income	1	1
Leased real estate investments
Schedule of Operating Leases by Property Type [Line Items]
Operating Lease, Lease Income	$ 92	$ 104