Unit-Based Compensation (Phantom Common Unit Awards) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended						9 Months Ended	12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unit-based compensation	$ 4	$ 3	$ 3	$ 4	$ 3	$ 3
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Non-vested at beginning of the period, Weighted Average Grant Date Fair Value (in dollars per share)			$ 29.15			$ 31.89	$ 29.15	$ 31.89
Granted, Weighted Average Grant Date Fair Value (in dollars per share)							29.01	27.67
Forfeited, Weighted Average Grant Date Fair Value (in dollars per share)							28.17	31.26
Non-vested at end of period, Weighted Average Grant Date Fair Value (in dollars per share)	$ 28.89						28.89	29.15
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value							$ 29.20	$ 32.92
Phantom common units [Member]
Nonvested, Number of Shares [Roll Forward]
Non-vested at beginning of period (in shares)			2,124,012			1,777,301	2,124,012	1,777,301
Granted (in shares)							41,311	1,072,600
Non-vested at end of period (in shares)	1,852,966						1,852,966	2,124,012
Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares							126,993	414,472
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares							(185,364)	(311,417)