Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating losses (federal and state)	$ 3	$ 3
Accrued expenses	10	8
Accrued workers' compensation costs	9	9
Stock based compensation	3	8
Tax benefits relating to uncertain positions	1	0
Tax credits (federal and state)	7	7
Total	33	35
Valuation allowance	(5)	(7)	$ (7)	$ (6)
Total deferred tax assets	28	28
Deferred tax liabilities:
Depreciation and amortization	(27)	(24)
Deferred service revenues	(41)	(62)
Prepaid health plan expenses	(1)	0
Prepaid commission expenses	(19)	(9)
Total deferred tax liabilities	(88)	(95)
Net deferred tax liabilities	$ (60)	$ (67)