Equity Method Investments - Summary of Balance Sheet and Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions		2 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mars
Statements of Income
Total revenues				$ 255.5	$ 229.8	$ 205.9
Total operating expenses				81.9	83.0	85.7
Operating income				173.6	146.8	120.2
Net income				173.6	146.8	120.3
Balance Sheets
Current assets	$ 40.0	$ 47.6	$ 40.9	47.6	40.0	40.9
Non-current assets	197.5	187.5	208.2	187.5	197.5	208.2
Total assets	237.5	235.1	249.1	235.1	237.5	249.1
Current liabilities	5.1	5.1	6.4	5.1	5.1	6.4
Non-current liabilities	0.0	0.0	0.0	0.0	0.0	0.0
Equity	232.4	230.0	242.7	230.0	232.4	242.7
Total liabilities and equity	237.5	235.1	249.1	235.1	237.5	249.1
Bengal
Statements of Income
Total revenues				72.8	69.5	70.3
Total operating expenses				28.1	28.7	28.1
Operating income				44.7	40.8	42.2
Net income				44.8	40.2	42.1
Balance Sheets
Current assets	34.0	25.0	29.0	25.0	34.0	29.0
Non-current assets	147.5	156.6	146.3	156.6	147.5	146.3
Total assets	181.5	181.6	175.3	181.6	181.5	175.3
Current liabilities	16.8	10.5	11.2	10.5	16.8	11.2
Non-current liabilities	0.7	0.3	0.9	0.3	0.7	0.9
Equity	164.0	170.8	163.2	170.8	164.0	163.2
Total liabilities and equity	181.5	181.6	175.3	181.6	181.5	175.3
LOCAP
Statements of Income
Total revenues				55.7	51.4	49.2
Total operating expenses				17.2	17.1	15.2
Operating income				38.5	34.3	34.0
Net income				26.5	20.8	20.9
Balance Sheets
Current assets	8.5	9.2	8.8	9.2	8.5	8.8
Non-current assets	46.6	60.4	41.3	60.4	46.6	41.3
Total assets	55.1	69.6	50.1	69.6	55.1	50.1
Current liabilities	38.8	37.2	36.7	37.2	38.8	36.7
Non-current liabilities	13.3	21.8	11.2	21.8	13.3	11.2
Equity	3.0	10.6	2.2	10.6	3.0	2.2
Total liabilities and equity	55.1	69.6	50.1	69.6	55.1	50.1
Poseidon
Statements of Income
Total revenues			0.0	117.1	120.3	123.7
Total operating expenses			0.0	32.6	30.7	28.6
Operating income			0.0	84.5	89.6	95.1
Net income			45.9	78.5	84.9
Balance Sheets
Current assets	17.1	18.7	18.5	18.7	17.1	18.5
Non-current assets	233.6	218.6	249.2	218.6	233.6	249.2
Total assets	250.7	237.3	267.7	237.3	250.7	267.7
Current liabilities	20.7	17.6	22.5	17.6	20.7	22.5
Non-current liabilities	219.7	237.4	203.5	237.4	219.7	203.5
Equity	10.3	(17.7)	41.7	(17.7)	10.3	41.7
Total liabilities and equity	250.7	237.3	$ 267.7	237.3	250.7	$ 267.7
Proteus
Statements of Income
Total revenues	0.3			30.8	24.7
Total operating expenses	0.1			13.4	11.7
Operating income	0.2			17.4	13.0
Net income	0.0			17.4
Balance Sheets
Current assets	24.0	54.8		54.8	24.0
Non-current assets	194.7	335.8		335.8	194.7
Total assets	218.7	390.6		390.6	218.7
Current liabilities	2.2	50.3		50.3	2.2
Non-current liabilities	70.3	206.2		206.2	70.3
Equity	146.2	134.1		134.1	146.2
Total liabilities and equity	218.7	390.6		390.6	218.7
Endymion
Statements of Income
Total revenues	0.3			33.4	28.1
Total operating expenses	0.2			13.6	12.3
Operating income	0.1			19.8	15.8
Net income	0.1			20.3
Balance Sheets
Current assets	10.0	8.9		8.9	10.0
Non-current assets	154.1	145.5		145.5	154.1
Total assets	164.1	154.4		154.4	164.1
Current liabilities	2.9	2.7		2.7	2.9
Non-current liabilities	17.2	16.0		16.0	17.2
Equity	144.0	135.7		135.7	144.0
Total liabilities and equity	$ 164.1	154.4		154.4	$ 164.1
Permian Basin
Statements of Income
Total revenues		3.8		8.3
Total operating expenses		2.0		5.0
Operating income		1.8		3.3
Net income		1.8
Balance Sheets
Current assets		18.7		18.7
Non-current assets		83.6		83.6
Total assets		102.3		102.3
Current liabilities		8.7		8.7
Non-current liabilities		0.7		0.7
Equity		92.9		92.9
Total liabilities and equity		$ 102.3		$ 102.3