CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net income	$ 482.4	$ 391.8	$ 377.5
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion	45.9	45.0	43.1
Loss from disposition of fixed assets	(3.4)	0.1	0.2
Non-cash interest expense	0.9	0.4	2.7
Allowance oil reduction to net realizable value	5.5	0.3	0.0
Undistributed equity earnings	(5.8)	(6.5)	4.1
Changes in operating assets and liabilities
Accounts receivable	(6.8)	(7.4)	0.3
Allowance oil	(5.8)	(3.4)	(4.0)
Prepaid expenses and other assets	(3.7)	(6.8)	(0.1)
Accounts payable	(6.9)	8.3	0.3
Deferred revenue and other unearned income	(4.3)	6.0	7.9
Accrued liabilities	10.4	4.6	(2.8)
Net cash provided by operating activities	508.4	432.4	429.2
Cash flows from investing activities
Capital expenditures	(49.2)	(58.0)	(45.8)
Acquisitions from Parent	(481.6)	(420.5)	(172.8)
Third party acquisitions	0.0	(37.9)	(42.0)
Capital contribution on acquisition	(28.0)	(12.0)	0.0
Purchase price adjustment	0.0	0.3	0.0
Return of investment	48.2	18.2	15.8
April 2017 Divestiture	0.0	0.8	0.0
Other	0.3	0.0	0.0
Net cash used in investing activities	(510.3)	(509.1)	(244.8)
Cash flows from financing activities
Net proceeds from equity offerings	973.3	277.9	818.1
Borrowing under credit facilities	1,820.0	1,693.1	638.7
Repayment of credit facilities	(1,572.9)	(533.1)	(410.0)
Contributions from general partner	20.0	5.8	9.8
Proceeds from April 2017 Divestiture	0.0	20.2	0.0
Capital distributions to general partner	(738.4)	(1,034.5)	(896.3)
Distributions to noncontrolling interest	(16.4)	(18.9)	(30.2)
Distributions to unitholders and general partner	(423.1)	(267.8)	(179.9)
Net distributions to Parent	0.0	(65.6)	(120.0)
Other contributions from Parent	11.6	18.2	14.4
Credit facility issuance costs	(1.3)	(2.4)	(0.6)
Other	(0.6)	(0.6)	(0.1)
Net cash provided by (used in) financing activities	72.2	92.3	(156.1)
Net increase (decrease) in cash and cash equivalents	70.3	15.6	28.3
Cash at beginning of the year	137.7	122.1	93.8
Cash at end of the year	208.0	137.7	122.1
Non-cash investing and financing transactions
Change in asset retirement obligation	1.7	0.0	(1.0)
Change in accrued capital expenditures	2.4	(0.1)	(1.1)
Other non-cash contributions from Parent	1.9	0.0	0.2
Net assets not contributed to the Partnership	0.0	(5.1)	0.0
Other non-cash capital distributions to general partner	0.0	0.0	(7.1)
Other non-cash contribution from general partner	$ 0.0	$ 0.0	$ 7.1