Summary of Significant Accounting Policies and Other Matters - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Accounting Policies [Abstract]
Net income for common stock	$ 152	$ 188	$ 576	$ 616
Weighted average common shares outstanding – basic (shares)	328.3	310.8	325.2	310.6
Add: Incremental shares attributable to effect of potentially dilutive securities (shares)	0.9	1.1	0.9	1.1
Adjusted weighted average common shares outstanding – diluted (shares)	329.2	311.9	326.1	311.7
Net income per common share - basic (dollars per share)	$ 0.46	$ 0.60	$ 1.77	$ 1.98
Net income per common share - diluted (dollars per share)	$ 0.46	$ 0.60	$ 1.77	$ 1.98