Investments (Details) - Additional Information - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Proceeds from sales of real estate investments	$ 74	$ 23	$ 69
Gross gains on sales of real estate investments	23	10	7
Gross losses on sales of real estate investments	0	0	$ 0
Accumulated depreciation on real estate held for investment purposes	383	364
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2019	101
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2020	90
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2021	77
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2022	62
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2023	39
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2024 and thereafter	$ 75
Combined average days to maturity of short-term securities (in days)	54 days
Amortized cost of short-term securities (which approximates fair value)	$ 3,985	$ 4,895
Minimum percentage of equity interest that must be held by the Company in order for the investment in a VIE to be considered significant	20.00%

Combined average maturity (in days) on short-term securities.

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- Definition

The gross gain resulting from sales and other disposals of real estate owned for investment purposes.

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The gross loss resulting from sales and other disposals of real estate owned for investment purposes.

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The minimum percentage of equity interest that must be held by the Company in order for the investment in a VIE to be considered significant.

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Future rental payments receivable within one year of the balance sheet date under an operating lease.

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Future rental payments receivable within the fifth year from the balance sheet date under an operating lease.

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Future rental payments receivable within the fourth year from the balance sheet date under an operating lease.

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Future rental payments receivable within the third year from the balance sheet date under an operating lease.

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Future rental payments receivable within the second year from the balance sheet date under an operating lease.

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Future minimum lease payments receivable under operating leases for periods greater than five years following the balance sheet date.

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Amount of short-term investments classified as other.

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Cash received from the sale of real estate that is held for investment, that is, it is part of an investing activity during the period.

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The cumulative amount of depreciation for real estate property held for investment purposes.

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