Equity Method Investments - Summary of Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Bengal
Schedule of Equity Method Investments [Line Items]
Total Revenues	$ 18.1	$ 17.6	$ 35.9	$ 35.0
Total Operating Expenses	7.1	7.1	14.3	13.5
Operating Income	11.0	10.5	21.6	21.5
Net Income	10.9	10.4	21.5	21.4
Odyssey
Schedule of Equity Method Investments [Line Items]
Total Revenues	9.9		19.7
Total Operating Expenses	1.0		2.0
Operating Income	8.9		17.7
Net Income	8.9		17.7
Mars
Schedule of Equity Method Investments [Line Items]
Total Revenues	66.4	64.2	131.3	121.5
Total Operating Expenses	24.3	20.5	44.2	39.0
Operating Income	42.1	43.7	87.1	82.5
Net Income	42.1	43.7	87.1	82.5
Poseidon
Schedule of Equity Method Investments [Line Items]
Total Revenues	28.5	32.0	57.4	59.4
Total Operating Expenses	8.5	7.7	16.6	14.3
Operating Income	20.0	24.3	40.8	45.1
Net Income	18.6	$ 23.0	38.0	$ 42.7
Proteus
Schedule of Equity Method Investments [Line Items]
Total Revenues	7.3		15.2
Total Operating Expenses	3.0		6.1
Operating Income	4.3		9.1
Net Income	4.0		8.5
Endymion
Schedule of Equity Method Investments [Line Items]
Total Revenues	8.5		17.1
Total Operating Expenses	3.2		6.3
Operating Income	5.3		10.8
Net Income	$ 4.8		$ 10.0