Stock-based Compensation (Summary of Restricted Stock Awards Vested) (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Payment Arrangement, Noncash Expense [Abstract]
Total shares vested	1.0	1.0	1.0
Fair value on vesting date	$ 135	$ 107	$ 67

Aggregate value of stock related to Restricted Stock Awards issued during the period.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration