Earnings Per Share Attributable to Stockholders (Narrative) (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Performance Share Units [Member]
Potential common share excluded from computation of diluted earnings per share (shares)	13,528	0
Stock Options [Member]
Potential common share excluded from computation of diluted earnings per share (shares)	1,619,141	0
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%