Earnings Per Share Attributable to Stockholders (Narrative) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance Share Units [Member]
Potential common share excluded from computation of diluted earnings per share (shares)		173,754	0
Stock Options [Member]
Potential common share excluded from computation of diluted earnings per share (shares)	668,900	752,197	253,839
Minimum [Member] | Performance Share Units [Member]
Number of units available for grant as a percentage of target	0.00%
Maximum [Member] | Performance Share Units [Member]
Number of units available for grant as a percentage of target	200.00%
Senior Executive and Employee Performance Share Unit Plans [Member] | Minimum [Member]
Number of units available for grant as a percentage of target	0.00%
Senior Executive and Employee Performance Share Unit Plans [Member] | Maximum [Member]
Number of units available for grant as a percentage of target	200.00%