Net Income per Common Share Attributable to Magellan Health, Inc. - Tabular Disclosure (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income	$ 18,250	$ 431
Denominator:
Weighted average number of common shares outstanding-basic	24,728	23,946
Common stock equivalents-stock options (in shares)	61	111
Common stock equivalents-RSAs (in shares)	21	7
Common stock equivalents-RSUs (in shares)	50	19
Common stock equivalents- PSUs( in shares)		124
Common stock equivalents-employee stock purchase plan (in shares)	9	6
Weighted average number of common shares outstanding-diluted (in shares)	24,869	24,213
Net income per common share-basic (in dollars per share)	$ 0.74	$ 0.02
Net income per common share-diluted (in dollars per share)	$ 0.73	$ 0.02