Business Segment Information - Operating Results by Business Segment (Details) - USD ($) $ 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
Operating results by business segment
Managed care and other revenue	$ 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 revenue	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
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	$ (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]
Stock compensation expense									39,116		37,422		50,384
Changes in fair value of contingent consideration									696		(104)		44,257
Impairment of intangible assets											4,800
Less: non-controlling interest segment profit (loss)									(59)		(737)		(2,634)
Segment profit (loss)									310,891		301,793		275,651
Operating segments | Healthcare
Operating results by business segment
Managed care and other revenue									3,206,277		2,659,685		2,959,252
Cost of care									(2,413,770)		(1,882,614)		(2,274,755)
Direct service costs and other									(601,201)		(573,706)		(510,811)
Stock compensation expense									10,689		4,440		8,502
Changes in fair value of contingent consideration									696		(231)		(1,404)
Impairment of intangible assets											4,800
Less: non-controlling interest segment profit (loss)									(56)		(567)		(2,439)
Segment profit (loss)									202,747		212,941		183,223
Operating segments | Pharmacy Management
Operating results by business segment
Managed care and other revenue									273,489		243,561		238,456
PBM and dispensing revenue									2,491,044		2,053,188		1,510,180
Cost of goods sold									(2,341,979)		(1,933,086)		(1,427,680)
Direct service costs and other									(302,525)		(261,570)		(284,968)
Stock compensation expense									19,881		20,509		36,351
Changes in fair value of contingent consideration											127		45,661
Segment profit (loss)									139,910		122,729		118,000
Corporate and Elimination
Operating results by business segment
Managed care and other revenue									(584)		(304)		(63)
PBM and dispensing revenue									(131,643)		(119,246)		(110,425)
Cost of goods sold									130,069		114,366		105,803
Direct service costs and other									(38,157)		(41,336)		(26,613)
Stock compensation expense									8,546		12,473		5,531
Less: non-controlling interest segment profit (loss)									(3)		(170)		(195)
Segment profit (loss)									$ (31,766)		$ (33,877)		$ (25,572)

[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.