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Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
7.25% 2063 Senior Notes
Financial Instruments
Interest rate	7.25%
7.25% 2064 Senior Notes
Financial Instruments
Interest rate	7.25%
6.95% Senior Notes
Financial Instruments
Interest rate	6.95%
6.25% Senior Notes
Financial Instruments
Interest rate	6.25%
5.5% Senior Notes
Financial Instruments
Interest rate	5.50%
6.7% Senior Notes
Financial Instruments
Interest rate	6.70%
Book Value | Retail
Financial Instruments
Long-term debt	$ 1,917	$ 1,917
Book Value | Institutional
Financial Instruments
Long-term debt	535	535
Book Value | Other
Financial Instruments
Long-term debt	598	106
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	1,963	1,962
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	661	707
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	$ 598	$ 106
Interest rate | Institutional and Other | Minimum
Financial Instruments
Fair value assumption, interest rate	1.31%	1.35%
Interest rate | Institutional and Other | Maximum
Financial Instruments
Fair value assumption, interest rate	4.48%	3.75%

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Contractual interest rate for funds borrowed, under the debt agreement.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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