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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	Feb. 29, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
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			144,046	244,219	143,658	243,197
Deferred stock units issued to executive officers	63,677	4,784
Deferred stock units issued to executive officers, aggregate fair value	$ 6.9	$ 0.3
Weighted Average
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred stock units issued to executive officers, exercise price	$ 108.47	$ 55.22