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Investments (Real Estate and Real Estate Joint Ventures) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Real Estate [Line Items]
Leased real estate investments, Carrying Value	$ 4,516		$ 4,516		$ 4,132
Other real estate investments, Carrying Value	488		488		461
Real estate joint ventures, Carrying Value	5,242		5,242		5,105
Real estate and real estate joint ventures (includes $250 and $0, respectively, of real estate held-for-sale)	10,246		10,246		$ 9,698
Real Estate and Real Estate Joint Ventures
Real Estate [Line Items]
Gross Investment Income, Operating	181	$ 175	490	$ 538
Leased real estate investments
Real Estate [Line Items]
Operating Lease, Lease Income	103	103	285	311
Other real estate investments
Real Estate [Line Items]
Operating Lease, Lease Income	45	45	135	135
Real estate joint ventures
Real Estate [Line Items]
Income (Loss) from Equity Method Investments	$ 33	$ 27	$ 70	$ 92