Stockholders' Equity - 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								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income (loss) from continuing operations	$ (24,696)	$ (17,296)	$ 47,108	$ (1,070)	$ 9,452	$ 4,121	$ 7,215	$ (8,191)	$ 4,046	$ 12,597	$ 22,176
Income from discontinued operations, net of tax	293,629	28,943	36,397	19,320	11,132	17,153	6,398	8,622	378,289	43,305	2,005
Net income	$ 268,933	$ 11,647	$ 83,505	$ 18,250	$ 20,584	$ 21,274	$ 13,613	$ 431	382,335	55,902	24,181
Net income available to common stockholders, basic									382,335	55,902	24,181
Net Income available to common stockholders, diluted									$ 382,335	$ 55,902	$ 24,181
Denominator:
Weighted average number of common shares outstanding-basic (in shares)	25,781	25,448	25,054	24,728	24,491	24,426	24,101	23,946	25,255	24,243	24,349
Weighted average number of common shares outstanding-diluted (in shares)	25,781	25,448	25,278	24,869	24,905	24,708	24,416	24,213	25,532	24,563	25,035
Net income per common share - basic:
Continuing operations (in dollars per share)	$ (0.96)	$ (0.68)	$ 1.88	$ (0.04)	$ 0.39	$ 0.17	$ 0.30	$ (0.34)	$ 0.16	$ 0.52	$ 0.91
Discontinued operations (in dollars per share)	11.39	1.14	1.45	0.78	0.45	0.70	0.26	0.36	14.98	1.79	0.08
Consolidated operations (in dollars per share)	10.43	0.46	3.33	0.74	0.84	0.87	0.56	0.02	15.14	2.31	0.99
Net income per common share - diluted:
Continuing operations (in dollars per share)	(0.96)	(0.68)	1.86	(0.04)	0.38	0.17	0.30	(0.34)	0.16	0.51	0.89
Discontinued operations (in dollars per share)	11.39	1.14	1.44	0.78	0.45	0.69	0.26	0.36	14.82	1.76	0.08
Consolidated operations (in dollars per share)	$ 10.43	$ 0.46	$ 3.30	$ 0.74	$ 0.83	$ 0.86	$ 0.56	$ 0.02	$ 14.98	$ 2.27	$ 0.97
Stock Options
Denominator:
Common stock equivalents (in shares)									79	142	493
Restricted Stock Awards
Denominator:
Common stock equivalents (in shares)									18	8	15
Restricted Stock Units (RSUs)
Denominator:
Common stock equivalents (in shares)									104	35	37
Performance Based Restricted Stock Units ("PSUs")
Denominator:
Common stock equivalents (in shares)									72	131	137
Employee Stock
Denominator:
Common stock equivalents (in shares)									4	4	4