Acquisitions - Acquisition of SWH Holdings (Details) - USD ($) $ in Thousands		2 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets acquired:
Goodwill		$ 1,006,288	$ 1,006,288	$ 742,054	$ 621,390
Liabilities assumed:
Medical claims payable		96,398	96,398
SWH
Acquisitions
Base purchase price	$ 400,000
Contingent payment included in purchase consideration	10,000
Total revenues subsequent to acquisition		186,600
Segment Profit subsequent to acquisition		9,500
Goodwill and identified intangible assets deductible for tax purposes	0
Assets acquired:
Current assets (includes $169,397 and $12,669 of cash and accounts receivable, respectively)	193,542
Cash	169,397
Accounts receivable	12,669
Property and equipment, net	3,395
Other assets	2,789
Identified intangible assets	124,140
Goodwill	260,139
Total assets acquired	584,005
Liabilities assumed:
Current liabilities (includes $96,398 of medical claims payable)	139,769
Medical claims payable	96,398
Deferred tax liabilities	45,913
Total liabilities assumed	185,682
Net assets acquired	398,323
Working capital receivable		$ 300	300
Acquisition related costs			$ 1,000
SWH | Trade names
Acquisitions
Indefinite-lived intangible assets acquired	30,000
SWH | Licenses
Acquisitions
Indefinite-lived intangible assets acquired	400
SWH | Customer contracts
Acquisitions
Finite-live intangible assets acquired	$ 86,500
SWH | Customer contracts | Minimum
Acquisitions
Period of amortization of intangible assets acquired	4 years
SWH | Customer contracts | Maximum
Acquisitions
Period of amortization of intangible assets acquired	6 years
SWH | Provider networks
Acquisitions
Finite-live intangible assets acquired	$ 7,200
Period of amortization of intangible assets acquired	3 years