Common Stockholders' Equity (Schedule of Changes in Shares of Common Stock Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock Shares Outstanding [Roll Forward]
Balance, beginning of year	183,411,907	182,993,640	182,272,278
Shares issued	776,544	687,339	1,257,762
Shares repurchased	(6,363,301)	(269,072)	(536,400)
Balance, end of year	177,825,150	183,411,907	182,993,640