Business Segment Information - Operating Results by Business Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating results by business segment
Net revenue	$ 1,184,960	$ 1,170,117	$ 1,100,075	$ 1,122,379	$ 1,133,775	$ 1,158,543	$ 1,154,289	$ 1,119,006	$ 4,577,531	$ 4,565,613	$ 4,957,522
Cost of care	(362,478)	(364,438)	(321,831)	(349,108)	(367,819)	(397,697)	(408,911)	(369,097)	(1,397,855)	(1,543,524)	(1,554,691)
Cost of goods sold	(559,140)	(560,269)	(528,067)	(533,241)	(507,917)	(523,973)	(501,081)	(526,314)	(2,180,717)	(2,059,285)	(2,452,703)
Direct service costs and other	(259,401)	(216,770)	(199,756)	(204,241)	(207,616)	(195,844)	(195,907)	(202,300)	(880,168)	(801,667)	(773,915)
Stock compensation expense									25,172	24,673	28,936
Changes in fair value of contingent consideration											1,108
Segment Profit (Loss)									143,963	185,810	206,257
Managed care and other
Operating results by business segment
Net revenue	579,454	568,688	548,711	553,168	580,544	591,229	608,614	566,548	2,250,021	2,346,935	2,350,576
PBM
Operating results by business segment
Net revenue	$ 605,506	$ 601,429	$ 551,364	$ 569,211	$ 553,231	$ 567,314	$ 545,675	$ 552,458	2,327,510	2,218,678	2,606,946
Healthcare
Operating results by business segment
Net revenue									1,959,869	2,082,088	2,110,756
Pharmacy Management
Operating results by business segment
Net revenue									2,638,301	2,502,268	2,865,844
Operating Segments | Healthcare
Operating results by business segment
Cost of care									(1,397,855)	(1,543,524)	(1,554,691)
Direct service costs and other									(433,723)	(402,006)	(401,083)
Stock compensation expense									6,876	7,639	6,446
Changes in fair value of contingent consideration											1,108
Segment Profit (Loss)									135,167	144,197	162,536
Operating Segments | Healthcare | Managed care and other
Operating results by business segment
Net revenue									1,959,869	2,082,088	2,110,756
Operating Segments | Pharmacy Management
Operating results by business segment
Cost of goods sold									(2,199,674)	(2,076,509)	(2,468,170)
Direct service costs and other									(360,970)	(323,162)	(298,713)
Stock compensation expense									7,723	7,834	5,458
Segment Profit (Loss)									85,380	110,431	104,419
Operating Segments | Pharmacy Management | Managed care and other
Operating results by business segment
Net revenue									290,855	265,439	240,427
Operating Segments | Pharmacy Management | PBM
Operating results by business segment
Net revenue									2,347,446	2,236,829	2,625,417
Corporate and Eliminations
Operating results by business segment
Cost of goods sold									18,957	17,224	15,467
Direct service costs and other									(85,475)	(76,499)	(74,119)
Stock compensation expense									10,573	9,200	17,032
Segment Profit (Loss)									(76,584)	(68,818)	(60,698)
Corporate and Eliminations | Managed care and other
Operating results by business segment
Net revenue									(703)	(592)	(607)
Corporate and Eliminations | PBM
Operating results by business segment
Net revenue									$ (19,936)	$ (18,151)	$ (18,471)