Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Net revenue:
Managed care and other	$ 1,093,785	$ 834,358	$ 821,699	$ 729,340	$ 775,031	$ 751,589	$ 699,861	$ 676,461	$ 3,479,182		$ 2,902,942		$ 3,197,645
PBM and dispensing	600,630	585,048	597,440	576,283	488,354	540,543	464,484	440,561	2,359,401		1,933,942		1,399,755
Total net revenue	1,694,415	1,419,406	1,419,139	1,305,623	1,263,385	1,292,132	1,164,345	1,117,022	5,838,583		4,836,884		4,597,400
Costs and expenses:
Cost of care	779,146	569,306	583,264	482,054	472,211	480,243	472,529	457,631	2,413,770		1,882,614		2,274,755
Cost of goods sold	563,240	543,682	562,355	542,633	456,658	509,673	436,930	415,459	2,211,910		1,818,720		1,321,877
Direct service costs and other operating expenses	261,653	227,372	231,372	221,486	240,985	229,094	214,077	192,456	941,883	[1],[2],[3]	876,612	[1],[2],[3]	822,392	[1],[2],[3]
Depreciation and amortization	32,810	28,189	27,731	26,976	28,574	26,885	25,580	25,007	115,706		106,046		102,844
Interest expense	9,266	7,663	4,900	4,148	3,413	3,038	1,994	1,748	25,977		10,193		6,581
Interest and other income	(2,086)	(1,781)	(1,071)	(949)	(702)	(741)	(692)	(683)	(5,887)		(2,818)		(2,165)
Total costs and expenses	1,644,029	1,374,431	1,408,551	1,276,348	1,201,139	1,248,192	1,150,418	1,091,618	5,703,359		4,691,367		4,526,284
Income (loss) before income taxes	50,386	44,975	10,588	29,275	62,246	43,940	13,927	25,404	135,224		145,517		71,116
Provision for income taxes	(4,123)	11,739	5,661	11,806	26,469	18,631	12,615	12,013	25,083		69,728		42,409
Net income	54,509	33,236	4,927	17,469	35,777	25,309	1,312	13,391	110,141		75,789		28,707
Less: net income (loss) attributable to non-controlling interest		785	(573)	(278)	602	(200)	(2,646)	154	(66)		(2,090)		(2,706)
Net income attributable to Magellan	$ 54,509	$ 32,451	$ 5,500	$ 17,747	$ 35,175	$ 25,509	$ 3,958	$ 13,237	$ 110,207		$ 77,879		$ 31,413
Weighted average number of common shares outstanding-basic	23,921	23,282	23,108	23,012	22,556	23,052	23,516	23,631	23,333		23,181		24,865
Weighted average number of common shares outstanding-diluted	25,113	24,563	24,038	24,038	23,493	24,009	24,643	24,511	24,440		24,156		25,877
Net income per common share-basic (in dollars per share)	$ 2.28	$ 1.39	$ 0.24	$ 0.77	$ 1.56	$ 1.11	$ 0.17	$ 0.56	$ 4.72		$ 3.36		$ 1.26
Net income per common share-diluted (in dollars per share)	$ 2.17	$ 1.32	$ 0.23	$ 0.74	$ 1.50	$ 1.06	$ 0.16	$ 0.54	$ 4.51		$ 3.22		$ 1.21
Stock compensation expense	$ 7,282	$ 10,323	$ 11,371	$ 10,140	$ 9,849	$ 9,176	$ 9,510	$ 8,887	$ 39,116		$ 37,422		$ 50,384
Changes in fair value of contingent consideration	$ 1,327	$ (834)	$ 252	$ (49)	$ (614)	$ 313	463	$ (266)	$ 696		(104)		$ 44,257
Impairment of intangible assets							$ 4,800				$ 4,800

[1] Includes changes in fair value of contingent consideration of $44,257, $(104) and $696 for the years ended December 31, 2015, 2016 and 2017, respectively.

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

[3] Includes stock compensation expense of $50,384, $37,422 and $39,116 for the years ended December 31, 2015, 2016 and 2017, respectively.