Summary of Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Basic earnings per share
Net income attributable to common shareholders	$ 318	$ 318	$ 577	$ 501
Net income available for common stock	$ 318	$ 318	$ 577	$ 501
Weighted average common shares outstanding (in shares)	380,000,000	375,000,000	379,000,000	367,000,000
Basic earnings per share (in dollars per share)	$ 0.84	$ 0.85	$ 1.52	$ 1.37
Diluted earnings per share
Net income attributable to common shareholders	$ 318	$ 318	$ 577	$ 501
Net income available for common stock	318	318	577	501
Net income available to common shareholders and assumed conversions	$ 318	$ 318	$ 577	$ 501
Weighted average common shares outstanding (in shares)	380,000,000	375,000,000	379,000,000	367,000,000
Incremental shares from assumed conversions (in shares)		1,000,000	1,000,000	1,000,000
Adjusted weighted average shares - diluted (in shares)	380,000,000	376,000,000	380,000,000	368,000,000
Diluted earnings per share (in dollars per share)	$ 0.84	$ 0.85	$ 1.52	$ 1.36
Antidilutive stock option awards excluded from earnings per share, number of shares	11,327,374	9,187,178	11,369,725	8,587,661