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

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Debt Obligations (3.25% Senior Unsecured Notes Due 2050) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2020	Mar. 31, 2020	Mar. 31, 2019	Apr. 30, 2019
Debt Instrument [Line Items]
Net proceeds from senior note		$ 644
Prepayment on revolving credit facility		$ 671	$ 515
3.25% Senior Unsecured Notes Due 2050 \| Senior Notes
Debt Instrument [Line Items]
Stated rate				3.25%
3.25% Senior Unsecured Notes Due 2050 \| Senior Notes \| Subsequent Event
Debt Instrument [Line Items]
Stated rate	3.25%
Net proceeds from senior note	$ 485
3.25% Senior Unsecured Notes Due 2050 \| Senior Notes \| Subsequent Event \| Maximum
Debt Instrument [Line Items]
Stated rate	5.25%