Stockholders' Equity - Calculation of Net Income per Common Share (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
Numerator:
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
Denominator:
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
Common stock equivalents-stock options (in shares)									530	289	316
Common stock equivalents-RSAs (in shares)									376	593	626
Common stock equivalents-RSUs (in shares)									64	45	33
Common stock equivalents-PSUs (in shares)									134	45	35
Common stock equivalents-employee stock purchase plan (in shares)									3	3	2
Weighted average number of common shares outstanding-diluted (in shares)	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
Potential dilutive securities excluded from computation of dilutive securities (in shares)									400	1,500	1,300