Consolidated Statements of Retained Earnings - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Partners Capital [Abstract]
Beginning Balance	$ 335	$ 311	$ 314
Add: Net income	341	316	286
Retained Earning, Gross	676	627	600
Deduct: Cash dividends on capital stock—common	362	292	289
Ending Balance	$ 314	$ 335	$ 311