Equity Method Investments - Summary of Income Statement Data for Equity Method Investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Bengal
Schedule of Equity Method Investments [Line Items]
Total Revenues	$ 17.8	$ 17.4
Total Operating Expenses	7.2	6.4
Operating Income	10.6	11.0
Net Income	10.6	11.0
Odyssey
Schedule of Equity Method Investments [Line Items]
Total Revenues	9.8
Total Operating Expenses	1.0
Operating Income	8.8
Net Income	8.8
Mars
Schedule of Equity Method Investments [Line Items]
Total Revenues	64.9	57.3
Total Operating Expenses	19.9	18.5
Operating Income	45.0	38.8
Net Income	45.0	38.8
Poseidon
Schedule of Equity Method Investments [Line Items]
Total Revenues	28.9	27.4
Total Operating Expenses	8.1	6.6
Operating Income	20.8	20.8
Net Income	19.4	$ 19.7
Proteus
Schedule of Equity Method Investments [Line Items]
Total Revenues	7.9
Total Operating Expenses	3.1
Operating Income	4.8
Net Income	4.5
Endymion
Schedule of Equity Method Investments [Line Items]
Total Revenues	8.6
Total Operating Expenses	3.1
Operating Income	5.5
Net Income	$ 5.2