Equity Method Investments - Summary of Balance Sheet and Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions		12 Months Ended
	May 11, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Amberjack
Statements of Income
Total revenues	$ 294.9	$ 204.0
Total operating expenses	73.5	47.3
Operating income	$ 221.4	156.7
Net income		156.8
Balance Sheets
Current assets		45.8
Non-current assets		846.2
Total assets		892.0
Current liabilities		4.3
Non-current liabilities		4.2
Equity (Deficit)		883.5
Total liabilities and equity (deficit)		892.0
Mars
Statements of Income
Total revenues		241.3	$ 255.5	$ 229.8
Total operating expenses		87.4	81.9	83.0
Operating income		153.9	173.6	146.8
Net income		153.9	173.6	146.8
Balance Sheets
Current assets		53.1	47.6	40.0
Non-current assets		178.2	187.5	197.5
Total assets		231.3	235.1	237.5
Current liabilities		5.4	5.1	5.1
Non-current liabilities		18.4	0.0	0.0
Equity (Deficit)		207.5	230.0	232.4
Total liabilities and equity (deficit)		231.3	235.1	237.5
Bengal
Statements of Income
Total revenues		69.2	72.8	69.5
Total operating expenses		28.1	28.1	28.7
Operating income		41.1	44.7	40.8
Net income		41.1	44.8	40.2
Balance Sheets
Current assets		27.1	25.0	34.0
Non-current assets		155.4	156.6	147.5
Total assets		182.5	181.6	181.5
Current liabilities		8.7	10.5	16.8
Non-current liabilities		0.0	0.3	0.7
Equity (Deficit)		173.8	170.8	164.0
Total liabilities and equity (deficit)		182.5	181.6	181.5
Poseidon
Statements of Income
Total revenues		115.5	117.1	120.3
Total operating expenses		34.6	32.6	30.7
Operating income		80.9	84.5	89.6
Net income		73.0	78.5	84.9
Balance Sheets
Current assets		18.9	18.7	17.1
Non-current assets		203.0	218.6	233.6
Total assets		221.9	237.3	250.7
Current liabilities		15.9	17.6	20.7
Non-current liabilities		242.9	237.4	219.7
Equity (Deficit)		(36.9)	(17.7)	10.3
Total liabilities and equity (deficit)		221.9	237.3	250.7
Other
Statements of Income
Total revenues		152.2	123.7	52.0
Total operating expenses		67.0	46.2	17.4
Operating income		85.2	77.5	34.6
Net income		75.8	66.0	20.9
Balance Sheets
Current assets		49.7	91.6	42.5
Non-current assets		875.7	625.3	395.4
Total assets		925.4	716.9	437.9
Current liabilities		64.5	98.9	43.9
Non-current liabilities		455.5	244.7	100.8
Equity (Deficit)		405.4	373.3	293.2
Total liabilities and equity (deficit)		$ 925.4	716.9	437.9
Proteus
Statements of Income
Total revenues				24.7
Total operating expenses				11.7
Operating income				13.0
Endymion
Statements of Income
Total revenues				28.1
Total operating expenses				12.3
Operating income				$ 15.8
Permian Basin
Statements of Income
Total revenues			8.3
Total operating expenses			5.0
Operating income			$ 3.3