Net Income per Common Share Attributable to Magellan Health, Inc (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 431	$ 11,452
Denominator:
Weighted average number of common shares outstanding-basic (in shares)	23,946	24,349
Common stock equivalents-stock options (in shares)	111	806
Common stock equivalents-RSAs (in shares)	7	24
Common stock equivalents-RSUs (in shares)	19	77
Common stock equivalents- PSUs( in shares)	124	353
Common stock equivalents-employee stock purchase plan (in shares)	6	3
Weighted average number of common shares outstanding-diluted (in shares)	24,213	25,612
Net income per common share-basic (in dollars per share)	$ 0.02	$ 0.47
Net income per common share-diluted (in dollars per share)	$ 0.02	$ 0.45
Potential dilutive securities excluded from computation of dilutive securities (in shares)	1,200	200