Net Income per Common Share Attributable to Magellan Health, Inc (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income attributable to Magellan	$ 27,145	$ 32,451	$ 52,148	$ 55,698
Denominator:
Weighted average number of common shares outstanding-basic (in shares)	24,433	23,282	24,451	23,135
Common stock equivalents-stock options (in shares)	413	572	630	485
Common stock equivalents-RSAs (in shares)	2	483	19	461
Common stock equivalents-RSUs (in shares)	26	61	42	46
Common stock equivalents- PSUs( in shares)	49	162	171	111
Common stock equivalents-employee stock purchase plan (in shares)	5	3	3	3
Weighted average number of common shares outstanding-diluted (in shares)	24,928	24,563	25,316	24,241
Net income per common share-basic (in dollars per share)	$ 1.11	$ 1.39	$ 2.13	$ 2.41
Net income per common share-diluted (in dollars per share)	$ 1.09	$ 1.32	$ 2.06	$ 2.30
Potential dilutive securities excluded from computation of dilutive securities (in shares)	600	400	400	600