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Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 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		463,711	144,046	463,479	143,658
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