Common Stockholders' Equity (Schedule of Changes in Shares of Common Stock Outstanding) (Details) - shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Common Stock Shares Outstanding [Roll Forward]
Balance, beginning of year	126,748,836	132,233,167	136,017,732
Shares issued	1,061,026	332,137	139,790
Shares repurchased	(4,502,025)	(5,816,468)	(3,924,355)
Balance, end of year	123,307,837	126,748,836	132,233,167