Investments (Details) - Additional Information - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments [Abstract]
Proceeds from sales of real estate investments	$ 0	$ 0	$ 74,000,000
Gross gains on sales of real estate investments			23,000,000
Gross losses on sales of real estate investments			$ 0
Accumulated depreciation on real estate held for investment purposes	462,000,000	422,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2021	115,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2022	101,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2023	75,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2024	58,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2025	38,000,000
Future minimum rental income expected on operating leases relating to the Company's real estate properties for 2026 and thereafter	$ 75,000,000
Combined average days to maturity of short-term securities (in days)	45 days
Amortized cost of short-term securities (which approximates fair value)	$ 5,511,000,000	$ 4,943,000,000
Availability of financial information provided by hedge funds following the date of the reporting	1 month
Availability of financial information provided by private equity and real estate partnerships following the date of the reporting, maximum	3 months
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%

Number of months within which financial information provided by hedge funds is available following the reporting date.

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Maximum number of months before which financial information provided by private equity and real estate partnerships is available following the reporting date.

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

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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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