Employee Retirement Plans - Summary of Fair Value of Pension Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	$ 187.4	$ 162.8	$ 190.2
Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	183.9	160.0		$ 3,600.0
Equities | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	23.9	29.6
Liability hedging | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	53.6	52.5
Hedge funds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	54.8	42.8
Corporate bonds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	38.6	25.8
Property | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	11.4	8.6
Other | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	1.6	0.7
Level 1 | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.9	0.4
Level 1 | Equities | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.0	0.0
Level 1 | Liability hedging | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.0	0.0
Level 1 | Hedge funds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.0	0.0
Level 1 | Corporate bonds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.0	0.0
Level 1 | Property | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.0	0.0
Level 1 | Other | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.9	0.4
Level 2 | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	183.0	159.6
Level 2 | Equities | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	23.9	29.6
Level 2 | Liability hedging | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	53.6	52.5
Level 2 | Hedge funds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	54.8	42.8
Level 2 | Corporate bonds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	38.6	25.8
Level 2 | Property | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	11.4	8.6
Level 2 | Other | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.7	0.3
Level 3 | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	3.5	2.8	$ 3.2
Level 3 | Equities | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.3	0.3
Level 3 | Liability hedging | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	2.0	1.6
Level 3 | Hedge funds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.5	0.4
Level 3 | Corporate bonds | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.5	0.4
Level 3 | Property | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	0.2	0.1
Level 3 | Other | Merchant Navy Officers Pension Fund
Defined Benefit Plan Disclosure [Line Items]
Market value of defined benefit plan assets	$ 0.0	$ 0.0