Equity Method Investments - Summary of Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Bengal
Schedule of Equity Method Investments [Line Items]
Total Revenues	$ 18.7	$ 17.3	$ 54.6	$ 52.3
Total Operating Expenses	6.8	7.2	21.1	20.7
Operating Income	11.9	10.1	33.5	31.6
Net Income	11.9	10.3	33.4	31.7
Odyssey
Schedule of Equity Method Investments [Line Items]
Total Revenues	11.1		30.8
Total Operating Expenses	1.0		3.0
Operating Income	10.1		27.8
Net Income	10.1		27.8
Mars
Schedule of Equity Method Investments [Line Items]
Total Revenues	66.0	54.0	197.3	175.5
Total Operating Expenses	19.5	23.1	63.7	62.1
Operating Income	46.5	30.9	133.6	113.4
Net Income	46.5	30.9	133.6	113.4
Poseidon
Schedule of Equity Method Investments [Line Items]
Total Revenues	30.6	31.3	88.0	90.7
Total Operating Expenses	8.2	8.2	24.8	22.5
Operating Income	22.4	23.1	63.2	68.2
Net Income	20.7	$ 22.0	58.7	$ 64.7
Proteus
Schedule of Equity Method Investments [Line Items]
Total Revenues	7.6		22.8
Total Operating Expenses	2.9		9.0
Operating Income	4.7		13.8
Net Income	4.4		12.9
Endymion
Schedule of Equity Method Investments [Line Items]
Total Revenues	8.2		25.3
Total Operating Expenses	3.3		9.6
Operating Income	4.9		15.7
Net Income	$ 4.9		$ 14.9