Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Net revenue:
Net revenue	$ 1,844,645	$ 1,853,506	$ 1,810,923	$ 1,805,077	$ 1,694,415	$ 1,419,406	$ 1,419,139	$ 1,305,623	$ 7,314,151		$ 5,838,583		$ 4,836,884
Costs and expenses:
Cost of care	959,906	938,031	935,814	928,661	779,146	569,306	583,264	482,054	3,762,412		2,413,770		1,882,614
Cost of goods sold	593,793	571,145	558,419	559,665	563,240	543,682	562,355	542,633	2,283,022		2,211,910		1,818,720
Direct service costs and other operating expenses	277,835	265,471	259,152	269,077	261,653	227,372	231,372	221,486	1,071,535	[1],[2],[3]	941,883	[1],[2],[3]	876,612	[1],[2],[3]
Depreciation and amortization	35,358	33,047	33,848	30,407	32,810	28,189	27,731	26,976	132,660		115,706		106,046
Interest expense	9,362	8,990	8,678	8,366	9,266	7,663	4,900	4,148	35,396		25,977		10,193
Interest and other income	(4,090)	(4,139)	(3,363)	(2,476)	(2,086)	(1,781)	(1,071)	(949)	(14,068)		(5,887)		(2,818)
Total costs and expenses	1,872,164	1,812,545	1,792,548	1,793,700	1,644,029	1,374,431	1,408,551	1,276,348	7,270,957		5,703,359		4,691,367
Income (loss) before income taxes	(27,519)	40,961	18,375	11,377	50,386	44,975	10,588	29,275	43,194		135,224		145,517
Provision (benefit) for income taxes	448	13,816	4,824	(75)	(4,123)	11,739	5,661	11,806	19,013		25,083		69,728
Net income					54,509	33,236	4,927	17,469	24,181		110,141		75,789
Less: net income (loss) attributable to non-controlling interest						785	(573)	(278)			(66)		(2,090)
Net income (loss) attributable to Magellan	$ (27,967)	$ 27,145	$ 13,551	$ 11,452	$ 54,509	$ 32,451	$ 5,500	$ 17,747	$ 24,181		$ 110,207		$ 77,879
Weighted average number of common shares outstanding-basic	24,048	24,433	24,569	24,349	23,921	23,282	23,108	23,012	24,349		23,333		23,181
Weighted average number of common shares outstanding-diluted	24,048	24,928	25,407	25,612	25,113	24,563	24,038	24,038	25,035		24,440		24,156
Net income (loss) per common share-basic (in dollars per share)	$ (1.16)	$ 1.11	$ 0.55	$ 0.47	$ 2.28	$ 1.39	$ 0.24	$ 0.77	$ 0.99		$ 4.72		$ 3.36
Net income (loss) per common share-diluted (in dollars per share)	$ (1.16)	$ 1.09	$ 0.53	$ 0.45	$ 2.17	$ 1.32	$ 0.23	$ 0.74	$ 0.97		$ 4.51		$ 3.22
Stock compensation expense	$ 2,067	$ 9,320	$ 10,439	$ 7,646	$ 7,282	$ 10,323	$ 11,371	$ 10,140	$ 29,472		$ 39,116		$ 37,422
Changes in fair value of contingent consideration	856	148	70	233	1,327	(834)	252	(49)	1,307		696		(104)
Managed care and other
Net revenue:
Net revenue	1,207,552	1,235,787	1,215,340	1,219,763	1,093,785	834,358	821,699	729,340	4,878,442		3,479,182		2,902,942
PBM
Net revenue:
Net revenue	$ 637,093	$ 617,719	$ 595,583	$ 585,314	$ 600,630	$ 585,048	$ 597,440	$ 576,283	$ 2,435,709		$ 2,359,401		$ 1,933,942

[1] Includes changes in fair value of contingent consideration of $(104), $696 and $1,307 for the years ended December 31, 2016, 2017 and 2018, respectively.

[2] Includes impairment of intangible assets of $4,800 for the year ended December 31, 2016.

[3] Includes stock compensation expense of $37,422, $39,116 and $29,472 for the years ended December 31, 2016, 2017 and 2018, respectively.