Acquisitions and Divestiture - Cash Flow Statement (Details) - USD ($)		3 Months Ended										12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Cash flows from operating activities
Net income		$ 102,500,000	$ 99,500,000	$ 91,700,000	$ 98,100,000		$ 95,500,000		$ 84,400,000	$ 98,600,000	$ 99,000,000	$ 391,800,000	[1]	$ 377,500,000	[1],[2],[3]	$ 374,000,000	[1],[2],[3]
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion												45,000,000		43,100,000	[2],[3]	37,900,000	[2],[3]
Loss on disposition of fixed asset												100,000		200,000	[2],[3]	300,000	[2],[3]
Non-cash interest expense												400,000		2,700,000	[2]	0	[2]
Allowance oil reduction to net realizable value												300,000		0	[2]	2,700,000	[2]
Undistributed equity earnings												(6,500,000)		4,100,000	[2]	(4,300,000)	[2]
Changes in operating assets and liabilities
Accounts receivable												(7,400,000)		300,000	[2]	3,400,000	[2]
Allowance oil												(3,400,000)		(4,000,000)	[2]	(4,700,000)	[2]
Prepaid expenses and other assets														(100,000)		(700,000)
Accounts payable												8,300,000		300,000	[2]	(7,700,000)	[2]
Deferred revenue and other unearned income												6,000,000		7,900,000	[2]	(14,200,000)	[2]
Accrued liabilities												4,600,000		(2,800,000)	[2]	9,100,000	[2]
Net cash provided by operating activities												432,400,000		429,200,000	[2]	395,800,000	[2]
Cash flows from investing activities
Capital expenditures												(58,000,000)		(45,800,000)	[2]	(51,700,000)	[2]
Acquisitions from Parent												(420,500,000)		(172,800,000)	[2]	(179,800,000)	[2]
Third party acquisitions														(42,000,000)
Return of investment												18,200,000		15,800,000	[2]	12,500,000	[2]
Payment of pre-IPO distributions from investments to SPLC																(11,900,000)
Net cash used in investing activities												(509,100,000)		(244,800,000)	[2]	(230,900,000)	[2]
Cash flows from financing activities
Net proceeds from private equity placement												0		0	[2]	297,400,000	[2]
Net proceeds from public offerings												277,900,000		818,100,000	[2]	296,800,000	[2]
Borrowing under credit facility												1,693,100,000		638,700,000	[2]	458,200,000	[2]
Contributions from general partner												5,800,000		9,800,000	[2]	6,100,000	[2]
Repayment of credit facilities												(533,100,000)		(410,000,000)	[2]	0	[2]
Capital distributions to general partner												(1,034,500,000)		(896,300,000)	[2]	(1,002,100,000)	[2]
Distributions to noncontrolling interest												(18,900,000)		(30,200,000)	[2]	(73,200,000)	[2]
Distributions to unitholders and general partner												(267,800,000)		(179,900,000)	[2]	(96,500,000)	[2]
Other contributions from Parent												18,200,000		14,400,000	[2]	40,400,000	[2]
Credit facility issuance costs												(2,400,000)		(600,000)	[2]	(300,000)	[2]
Net distributions to Parent												(65,600,000)		(120,000,000)	[2]	(148,100,000)	[2]
Capital lease payments														(100,000)
Net cash provided by (used in) financing activities												92,300,000		(156,100,000)	[2]	(221,300,000)	[2]
Net increase (decrease) in cash and cash equivalents												15,600,000		28,300,000	[2]	(56,400,000)	[2]
Cash at beginning of the year	[2]				122,100,000	[4]					93,800,000	122,100,000	[4]	93,800,000		150,200,000
Cash at end of the year		$ 137,700,000					122,100,000	[2],[4]				137,700,000		122,100,000	[2],[4]	93,800,000	[2]
Non-cash investing activity
Increase in asset retirement obligation												0		(1,000,000)	[2]	(200,000)	[2]
Non-cash investing and financing transactions
Change in accrued capital expenditures												(100,000)		(1,100,000)	[2]	(6,500,000)	[2]
Contribution of fixed assets from Parent												0		0	[2]	400,000	[2]
Commencement of capital lease												0		0	[2]	22,800,000	[2]
Other non-cash contributions from Parent												0		200,000	[2]	1,800,000	[2]
Other non-cash capital distributions to general partner														(7,100,000)
Other non-cash contribution from general partner												0		7,100,000	[2]	0	[2]
Net assets not contributed to Parent																(6,400,000)
Scenario, Previously Reported
Cash flows from operating activities
Net income														265,400,000		272,900,000
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion														23,700,000		21,600,000
Loss on disposition of fixed asset														100,000		0
Non-cash interest expense														2,700,000
Allowance oil reduction to net realizable value																1,600,000
Undistributed equity earnings														1,800,000		(600,000)
Changes in operating assets and liabilities
Accounts receivable														100,000		0
Allowance oil														(4,800,000)		(2,800,000)
Prepaid expenses and other assets														(1,000,000)		(1,200,000)
Accounts payable														(1,500,000)		(4,500,000)
Deferred revenue and other unearned income														8,200,000		(13,800,000)
Accrued liabilities														(1,700,000)		9,500,000
Net cash provided by operating activities														293,000,000		282,700,000
Cash flows from investing activities
Capital expenditures														(28,300,000)		(16,200,000)
Acquisitions from Parent														(172,800,000)		(179,800,000)
Third party acquisitions														(42,000,000)
Return of investment														15,000,000		6,800,000
Payment of pre-IPO distributions from investments to SPLC																(11,900,000)
Net cash used in investing activities														(228,100,000)		(201,100,000)
Cash flows from financing activities
Net proceeds from private equity placement																297,400,000
Net proceeds from public offerings														818,100,000		296,800,000
Borrowing under credit facility														638,700,000		458,200,000
Contributions from general partner														9,800,000		6,100,000
Repayment of credit facilities														(410,000,000)
Capital distributions to general partner														(896,300,000)		(1,002,100,000)
Distributions to noncontrolling interest														(20,300,000)		(67,100,000)
Distributions to unitholders and general partner														(179,900,000)		(96,500,000)
Other contributions from Parent														4,600,000		11,100,000
Credit facility issuance costs														(600,000)		(300,000)
Net distributions to Parent														0		(42,400,000)
Capital lease payments														(100,000)
Net cash provided by (used in) financing activities														(36,000,000)		(138,800,000)
Net increase (decrease) in cash and cash equivalents														28,900,000		(57,200,000)
Cash at beginning of the year					121,900,000						93,000,000	121,900,000		93,000,000		150,200,000
Cash at end of the year							121,900,000							121,900,000		93,000,000
Non-cash investing activity
Increase in asset retirement obligation														0		0
Non-cash investing and financing transactions
Change in accrued capital expenditures														500,000		2,900,000
Contribution of fixed assets from Parent																400,000
Commencement of capital lease																22,800,000
Other non-cash contributions from Parent														200,000		1,800,000
Other non-cash capital distributions to general partner														(7,100,000)
Other non-cash contribution from general partner														7,100,000
Net assets not contributed to Parent																(6,400,000)
May 2017 Acquisition | Scenario, Previously Reported
Cash flows from operating activities
Net income														11,700,000		13,800,000
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion														12,900,000		10,200,000
Loss on disposition of fixed asset														0		0
Non-cash interest expense														0
Allowance oil reduction to net realizable value																1,100,000
Undistributed equity earnings														0		0
Changes in operating assets and liabilities
Accounts receivable														700,000		4,500,000
Allowance oil														800,000		(1,900,000)
Prepaid expenses and other assets														900,000		(800,000)
Accounts payable														1,100,000		(300,000)
Deferred revenue and other unearned income														(300,000)		(400,000)
Accrued liabilities														0		(100,000)
Net cash provided by operating activities														27,800,000		26,100,000
Cash flows from investing activities
Capital expenditures														(7,700,000)		(23,700,000)
Acquisitions from Parent														0		0
Third party acquisitions														0
Return of investment														0		0
Payment of pre-IPO distributions from investments to SPLC
Net cash used in investing activities														(7,700,000)		(23,700,000)
Cash flows from financing activities
Net proceeds from private equity placement																0
Net proceeds from public offerings														0		0
Borrowing under credit facility														0		0
Contributions from general partner														0		0
Repayment of credit facilities														0
Capital distributions to general partner														0		0
Distributions to noncontrolling interest														0		0
Distributions to unitholders and general partner														0		0
Other contributions from Parent														9,800,000		29,300,000
Credit facility issuance costs														0		0
Net distributions to Parent														(29,900,000)		(31,700,000)
Capital lease payments														0
Net cash provided by (used in) financing activities														(20,100,000)		(2,400,000)
Net increase (decrease) in cash and cash equivalents														0		0
Cash at beginning of the year					0						0	0		0		0
Cash at end of the year							0							0		0
Non-cash investing activity
Increase in asset retirement obligation														0		0
Non-cash investing and financing transactions
Change in accrued capital expenditures														(3,000,000)		(9,100,000)
Contribution of fixed assets from Parent																0
Commencement of capital lease																0
Other non-cash contributions from Parent														0		0
Other non-cash capital distributions to general partner														0
Other non-cash contribution from general partner														0
Net assets not contributed to Parent																(0.0)
December 2017 Acquisition | Scenario, Previously Reported
Cash flows from operating activities
Net income														100,400,000		87,300,000
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation, amortization and accretion														6,500,000		6,100,000
Loss on disposition of fixed asset														100,000		300,000
Non-cash interest expense														0
Allowance oil reduction to net realizable value																0
Undistributed equity earnings														2,300,000		(3,700,000)
Changes in operating assets and liabilities
Accounts receivable														(500,000)		(1,100,000)
Allowance oil														0		0
Prepaid expenses and other assets														0		1,300,000
Accounts payable														700,000		(2,900,000)
Deferred revenue and other unearned income														0		0
Accrued liabilities														(1,100,000)		(300,000)
Net cash provided by operating activities														108,400,000		87,000,000
Cash flows from investing activities
Capital expenditures														(9,800,000)		(11,800,000)
Acquisitions from Parent														0		0
Third party acquisitions														0
Return of investment														800,000		5,700,000
Payment of pre-IPO distributions from investments to SPLC
Net cash used in investing activities														(9,000,000)		(6,100,000)
Cash flows from financing activities
Net proceeds from private equity placement																0
Net proceeds from public offerings														0		0
Borrowing under credit facility														0		0
Contributions from general partner														0		0
Repayment of credit facilities														0
Capital distributions to general partner														0		0
Distributions to noncontrolling interest														(9,900,000)		(6,100,000)
Distributions to unitholders and general partner														0		0
Other contributions from Parent														0		0
Credit facility issuance costs														0		0
Net distributions to Parent														(90,100,000)		(74,000,000)
Capital lease payments														0
Net cash provided by (used in) financing activities														(100,000,000)		(80,100,000)
Net increase (decrease) in cash and cash equivalents														(600,000)		800,000
Cash at beginning of the year					$ 200,000						$ 800,000	$ 200,000		800,000		0
Cash at end of the year							$ 200,000							200,000		800,000
Non-cash investing activity
Increase in asset retirement obligation														(1,000,000)		(200,000)
Non-cash investing and financing transactions
Change in accrued capital expenditures														1,400,000		(300,000)
Contribution of fixed assets from Parent																0
Commencement of capital lease																0
Other non-cash contributions from Parent														0		0
Other non-cash capital distributions to general partner														0
Other non-cash contribution from general partner														$ 0
Net assets not contributed to Parent																$ 0

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[2] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[3] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.

[4] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.