Description of Business and Significant Accounting Policies - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) segment	Dec. 31, 2019 USD ($)
Product Information [Line Items]
Number of reportable segments | segment	1
Advance collections of unbilled revenues	$ 47
Prepayments offsetting accrued health insurance costs	$ 40	$ 39
Non-US | Revenue | Foreign Sales
Product Information [Line Items]
Percent of concentration risk	1.00%
Health Care
Product Information [Line Items]
Prepayments offsetting accrued health insurance costs	$ 58	$ 52