Related Party Debt - Additional Information (Details) - USD ($)											1 Months Ended	12 Months Ended
	Aug. 07, 2018	May 11, 2018	Feb. 06, 2018	Dec. 01, 2017	Sep. 15, 2017	May 10, 2017	Mar. 01, 2017	May 23, 2016	Mar. 29, 2016	Aug. 06, 2014	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018	Sep. 27, 2016
Line Of Credit Facility [Line Items]
Interest and fee expenses associated with borrowings												$ 61,000,000.0	$ 29,400,000	$ 8,600,000
Interest paid												53,200,000	25,000,000.0	7,000,000.0
Repayments of credit facilities												1,572,900,000	533,100,000	$ 410,000,000.0
Total Capacity											$ 2,390,000,000.0	2,990,000,000.0	2,390,000,000.0
May 2018 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred		$ 1,220,000,000.0
December 2017 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred				$ 825,000,000.0
May 2017 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred						$ 630,000,000.0
Consideration, cash on hand						50,000,000.0
May 2016 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred								$ 700,000,000.0
Consideration, cash on hand								50,400,000
Seven Year Fixed Facility | Line of Credit
Line Of Credit Facility [Line Items]
Payments of issuance fee	$ 1,300,000
Total Capacity											0	$ 600,000,000.0	0		$ 600,000,000.0
Fixed interest rate															4.06%
Seven Year Fixed Facility | Line of Credit | Affiliated Entity | STCW
Line Of Credit Facility [Line Items]
Debt instrument term												7 years
Carrying amount of total debt											1,858,400,000	$ 2,099,100,000	1,858,400,000
Five Year Revolver Due July 2023 | Line of Credit
Line Of Credit Facility [Line Items]
Interest paid				700,000
Debt instrument term												5 years
Repayments of credit facilities			$ 246,900,000		$ 265,000,000.0				$ 272,600,000		268,100,000
Unused capacity commitment fee percentage												0.19%
Total Capacity											760,000,000.0	$ 760,000,000.0	760,000,000.0
Interest rate												3.60%
Five Year Revolver Due July 2023 | Line of Credit | May 2018 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred		494,000,000.0
Five Year Revolver Due July 2023 | Line of Credit | May 2017 Acquisition
Line Of Credit Facility [Line Items]
Consideration, funded with borrowings						73,100,000
Five Year Revolver Due July 2023 | Line of Credit | May 2016 Acquisition
Line Of Credit Facility [Line Items]
Consideration, funded with borrowings								$ 296,700,000
Five Year Revolver Due July 2023 | Line of Credit | Affiliated Entity
Line Of Credit Facility [Line Items]
Increase in maximum borrowing capacity																$ 760,000,000.0
Credit facility issuance fee																$ 600,000
Five Year Revolver due December 2022
Line Of Credit Facility [Line Items]
Interest rate												3.30%
Five Year Revolver due December 2022 | Line of Credit
Line Of Credit Facility [Line Items]
Debt instrument term												5 years
Borrowings under credit facilities				1,000,000,000.0
Repayments of credit facilities			$ 726,000,000.0
Payments of issuance fee				1,700,000
Unused capacity commitment fee percentage												0.19%
Total Capacity											1,000,000,000.0	$ 1,000,000,000.0	1,000,000,000.0
Five Year Revolver due December 2022 | Line of Credit | May 2018 Acquisition
Line Of Credit Facility [Line Items]
Consideration transferred		$ 726,000,000.0
Five Year Revolver due December 2022 | Line of Credit | December 2017 Acquisition
Line Of Credit Facility [Line Items]
Consideration, funded with borrowings				825,000,000.0
Five Year Fixed Facility | Line of Credit
Line Of Credit Facility [Line Items]
Borrowings under credit facilities				$ 93,100,000
Total Capacity											600,000,000.0	$ 600,000,000.0	600,000,000.0
Five Year Fixed Facility | Line of Credit | May 2017 Acquisition
Line Of Credit Facility [Line Items]
Consideration, funded with borrowings						$ 506,900,000
Five Year Fixed Facility | Line of Credit | STCW
Line Of Credit Facility [Line Items]
Payments of issuance fee							$ 700,000
Total Capacity							$ 600,000,000.0
Five Year Fixed Facility | Line of Credit | Affiliated Entity
Line Of Credit Facility [Line Items]
Fixed interest rate							3.23%
Five Year Fixed Facility | Line of Credit | Affiliated Entity | STCW
Line Of Credit Facility [Line Items]
Debt instrument term												5 years
Carrying amount of total debt											1,846,900,000	$ 2,094,000,000.0	1,846,900,000
364 Day Revolver | Line of Credit
Line Of Credit Facility [Line Items]
Debt instrument term									364 days
Repayments of credit facilities									$ 137,400,000
Zydeco Revolver | Line of Credit
Line Of Credit Facility [Line Items]
Debt instrument term										6 months
Unused capacity commitment fee percentage												0.23%
Total Capacity										$ 30,000,000.0	$ 30,000,000.0	$ 30,000,000.0	$ 30,000,000.0
Fixed interest rate												3.90%