Summary of Significant Accounting Policies - Cost Method Investment (Details) - USD ($) $ in Millions	Dec. 31, 2017	Nov. 30, 2017	Dec. 31, 2016
Schedule of Cost-method Investments [Line Items]
Amount	$ 62.1		$ 62.1	[1]
Colonial
Schedule of Cost-method Investments [Line Items]
Ownership	6.00%		6.00%
Amount	$ 11.4		$ 11.4
Explorer
Schedule of Cost-method Investments [Line Items]
Ownership	12.62%	2.62%	12.62%
Amount	$ 48.6		$ 48.6
Cleopatra
Schedule of Cost-method Investments [Line Items]
Ownership	1.00%		1.00%
Amount	$ 2.1		$ 2.1

[1] The financial information presented has been retrospectively adjusted for acquisitions of businesses under common control.