MEDICAL BENEFITS PAYABLE - Reconciliation of Beginning and Ending Balances, by Segment (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the beginning and ending balance of medical benefits payable [Roll Forward]
Beginning balance	$ 2,897.4	$ 2,146.3
Acquisitions	0.0	528.0
Medical benefits incurred related to
Current year	18,324.6	12,416.2
Prior years	(440.1)	(393.2)
Total	17,884.5	12,023.0
Medical benefits paid related to
Current year	(15,428.3)	(10,448.2)
Prior years	(1,989.7)	(1,347.7)
Total	(17,418.0)	(11,795.9)
Ending balance	3,363.9	2,901.4
Premium deficiency reserve	0.0	20.6	$ 16.1	$ 45.6
Medicaid Health Plans
Reconciliation of the beginning and ending balance of medical benefits payable [Roll Forward]
Beginning balance	2,012.8	1,373.2
Acquisitions	0.0	478.2
Medical benefits incurred related to
Current year	12,885.1	7,803.2
Prior years	(285.6)	(202.1)
Total	12,599.5	7,601.1
Medical benefits paid related to
Current year	(10,936.0)	(6,562.2)
Prior years	(1,384.3)	(889.3)
Total	(12,320.3)	(7,451.5)
Ending balance	2,292.0	2,001.0
Medicare Health Plans
Reconciliation of the beginning and ending balance of medical benefits payable [Roll Forward]
Beginning balance	823.5	722.5
Acquisitions	0.0	47.1
Medical benefits incurred related to
Current year	4,732.1	4,051.6
Prior years	(104.9)	(121.8)
Total	4,627.2	3,929.8
Medical benefits paid related to
Current year	(3,858.4)	(3,382.3)
Prior years	(600.7)	(488.3)
Total	(4,459.1)	(3,870.6)
Ending balance	991.6	828.8
Medicare PDPs
Reconciliation of the beginning and ending balance of medical benefits payable [Roll Forward]
Beginning balance	59.1	50.6
Acquisitions	0.0	0.0
Medical benefits incurred related to
Current year	700.2	560.4
Prior years	(48.5)	(69.3)
Total	651.7	491.1
Medical benefits paid related to
Current year	(628.4)	(502.9)
Prior years	(3.8)	30.0
Total	(632.2)	(472.9)
Ending balance	78.6	68.8
Corporate and other
Reconciliation of the beginning and ending balance of medical benefits payable [Roll Forward]
Beginning balance	2.0	0.0
Acquisitions	0.0	2.7
Medical benefits incurred related to
Current year	7.2	1.0
Prior years	(1.1)	0.0
Total	6.1	1.0
Medical benefits paid related to
Current year	(5.5)	(0.8)
Prior years	(0.9)	(0.1)
Total	(6.4)	(0.9)
Ending balance	$ 1.7	$ 2.8

WELLCARE HEALTH PLANS, INC. 10-Q