Employee Retirement Plans - Summary of GMSL's Investments Plan (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	$ 187.4	$ 162.8	$ 190.2
Present value of liabilities	(205.9)	(181.3)	(208.7)
Net pension liability	(18.5)
Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	183.9	160.0		$ 2.8
Present value of liabilities	(202.7)	(178.6)
Net pension liability	(18.8)	(18.6)
Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	183.9	160.0		$ 3,600.0
Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	3.5	2.8	$ 3.2
Present value of liabilities	(3.1)	(2.8)
Net pension liability	0.4	0.0
Equities | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	23.9	29.6
Equities | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	23.9	29.6
Equities | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	0.3	0.3
Liability hedging | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	53.6	52.5
Liability hedging | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	53.6	52.5
Liability hedging | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	2.0	1.6
Hedge Funds | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	54.8	42.8
Hedge Funds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	54.8	42.8
Hedge Funds | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	0.5	0.4
Corporate bonds | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	38.6	25.8
Corporate bonds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	38.6	25.8
Corporate bonds | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	0.5	0.4
Property | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	11.4	8.6
Property | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	11.4	8.6
Property | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	0.2	0.1
Other | Global Marine Systems Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	1.6	0.7
Other | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	1.6	0.7
Other | Merchant Navy Officers Pension Fund | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total market value of assets	$ 0.0	$ 0.0