Fair Value Measurements (Fair value of financial instruments) (Details) - Senior notes - Level 1 - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 6,967	$ 6,965
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior notes	$ 7,344	$ 7,470

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant