Equity Method Investments - Summary of Balance Sheet and Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions		2 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Mars
Statements of Income
Total revenues		$ 0.0			$ 229.8	$ 205.9	$ 160.5
Total operating expenses		0.0			83.0	85.7	73.0
Operating income		0.0			146.8	120.3	87.4
Net income		11.9			146.8	120.3
Balance Sheets
Current assets	$ 40.0	38.3	$ 40.0	$ 40.9	40.0	40.9	38.3
Non-current assets	197.5	212.1	197.5	208.2	197.5	208.2	212.1
Total assets	237.5	250.4	237.5	249.1	237.5	249.1	250.4
Current liabilities	5.1	6.1	5.1	6.4	5.1	6.4	6.1
Non-current liabilities	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Equity	232.4	244.3	232.4	242.7	232.4	242.7	244.3
Total liabilities and equity	237.5	250.4	237.5	249.1	237.5	249.1	250.4
Bengal
Statements of Income
Total revenues		0.0			69.5	70.3	65.9
Total operating expenses		0.0			28.7	28.1	27.2
Operating income		0.0			40.8	42.2	38.7
Net income		6.6			40.2	42.1
Balance Sheets
Current assets	34.0	35.9	34.0	29.0	34.0	29.0	35.9
Non-current assets	147.5	146.7	147.5	146.3	147.5	146.3	146.7
Total assets	181.5	182.6	181.5	175.3	181.5	175.3	182.6
Current liabilities	16.8	19.0	16.8	11.2	16.8	11.2	19.0
Non-current liabilities	0.7	1.2	0.7	0.9	0.7	0.9	1.2
Equity	164.0	162.4	164.0	163.2	164.0	163.2	162.4
Total liabilities and equity	181.5	$ 182.6	181.5	175.3	181.5	175.3	$ 182.6
Poseidon
Statements of Income
Total revenues				0.0	120.3	123.7
Total operating expenses				0.0	30.7	28.6
Operating income				0.0	89.6	95.1
Net income				45.9	84.9
Balance Sheets
Current assets	17.1		17.1	18.5	17.1	18.5
Non-current assets	233.6		233.6	249.2	233.6	249.2
Total assets	250.7		250.7	267.7	250.7	267.7
Current liabilities	20.7		20.7	22.5	20.7	22.5
Non-current liabilities	219.7		219.7	203.5	219.7	203.5
Equity	10.3		10.3	41.7	10.3	41.7
Total liabilities and equity	250.7		250.7	$ 267.7	250.7	$ 267.7
Odyssey
Statements of Income
Total revenues			9.3		37.0
Total operating expenses			0.8		3.2
Operating income			8.5		33.8
Net income			8.5
Balance Sheets
Current assets	3.6		3.6		3.6
Non-current assets	6.9		6.9		6.9
Total assets	10.5		10.5		10.5
Current liabilities	0.2		0.2		0.2
Non-current liabilities	5.0		5.0		5.0
Equity	5.3		5.3		5.3
Total liabilities and equity	10.5		10.5		10.5
Proteus
Statements of Income
Total revenues	0.3				24.7
Total operating expenses	0.1				11.7
Operating income	0.2				13.0
Net income	0.0
Balance Sheets
Current assets	24.0		24.0		24.0
Non-current assets	194.7		194.7		194.7
Total assets	218.7		218.7		218.7
Current liabilities	2.2		2.2		2.2
Non-current liabilities	70.3		70.3		70.3
Equity	146.2		146.2		146.2
Total liabilities and equity	218.7		218.7		218.7
Endymion
Statements of Income
Total revenues	0.3				28.1
Total operating expenses	0.2				12.3
Operating income	0.1				15.8
Net income	0.1
Balance Sheets
Current assets	10.0		10.0		10.0
Non-current assets	154.1		154.1		154.1
Total assets	164.1		164.1		164.1
Current liabilities	2.9		2.9		2.9
Non-current liabilities	17.2		17.2		17.2
Equity	144.0		144.0		144.0
Total liabilities and equity	$ 164.1		$ 164.1		$ 164.1