Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Allowance for uncollectible accounts	$ 40	$ 41
Common stock, par value (dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	186,188,334	185,903,727
Treasury Stock, shares (in shares)	5,167,039	5,090,855