Common Stockholders' Equity (Schedule of Changes in Shares of Common Stock Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock Shares Outstanding [Roll Forward]
Balance, beginning of year	121,193,599	123,307,837
Shares issued	1,052,256	281,654	1,061,026
Shares repurchased	(731,003)	(2,395,892)	(4,502,025)
Balance, end of year	121,514,852	121,193,599	123,307,837
Common Stock [Member]
Common Stock Shares Outstanding [Roll Forward]
Balance, beginning of year	121,193,599	123,307,837	126,748,836
Balance, end of year		121,193,599	123,307,837