CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Common Stock In Treasury	Additional Paid in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Total
Balance at Dec. 31, 2014	$ 501	$ (1,064,963)	$ 1,018,266	$ 1,179,897	$ (143)	$ 1,133,558
Balance (in shares) at Dec. 31, 2014	50,085	(23,150)
Increase (Decrease) in Stockholders' Equity
Stock compensation expense			50,384			50,384
Exercise of stock options	$ 11		54,079			54,090
Exercise of stock options (in shares)	1,140
Tax benefit from exercise of stock options and vesting of stock awards			3,530			3,530
Issuance of equity	$ 1		408			409
Issuance of equity (in shares)	115
Repurchase of stock		$ (204,428)				(204,428)
Repurchase of stock (in shares)		(3,498)
Adjustment to additional paid in capital due to reversal of tax contingency			32			32
Adjustment to non-controlling interest			(2,686)			(2,686)
Net income attributable to Magellan				31,413		31,413
Other comprehensive income (loss)-other					(119)	(119)
Balance at Dec. 31, 2015	$ 513	$ (1,269,391)	1,124,013	1,211,310	(262)	1,066,183
Balance (in shares) at Dec. 31, 2015	51,340	(26,648)
Increase (Decrease) in Stockholders' Equity
Stock compensation expense			37,422			37,422
Exercise of stock options	$ 6		24,542			24,548
Exercise of stock options (in shares)	494
Adjustment due to adoption of ASU 2016-09				99		99
Issuance of equity	$ 1		1,229			1,230
Issuance of equity (in shares)	159
Repurchase of stock		$ (106,806)				(106,806)
Repurchase of stock (in shares)		(1,828)
Adjustment to non-controlling interest			(923)			(923)
Net income attributable to Magellan				77,879		77,879
Other comprehensive income (loss)-other					87	87
Balance at Dec. 31, 2016	$ 520	$ (1,376,197)	1,186,283	1,289,288	(175)	1,099,719
Balance (in shares) at Dec. 31, 2016	51,993	(28,476)
Increase (Decrease) in Stockholders' Equity
Stock compensation expense			39,116			39,116
Exercise of stock options	$ 8		44,347			44,355
Exercise of stock options (in shares)	831
Issuance of equity	$ 2		361			363
Issuance of equity (in shares)	149
Repurchase of stock		$ (21,765)				(21,765)
Repurchase of stock (in shares)		(295)
Adjustment to non-controlling interest			4,704			4,704
Net income attributable to Magellan				110,207		110,207
Other comprehensive income (loss)-other					(205)	(205)
Balance at Dec. 31, 2017	$ 530	$ (1,397,962)	$ 1,274,811	$ 1,399,495	$ (380)	$ 1,276,494
Balance (in shares) at Dec. 31, 2017	52,973	(28,771)