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Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2017	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average diluted shares outstanding excludes shares of common stock subject to stock options		447,625	143,270
Deferred stock units issued to executive officers	63,677
Deferred stock units issued to executive officers, aggregate fair value	$ 6.9
Weighted Average
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred stock units issued to executive officers, exercise price	$ 108.47