Employee Benefit Plans - Additional Information (Details)	Mar. 31, 2018 USD ($)
Post-Retirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Expected contributions	$ 329,000
Supplemental Employee Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Expected contributions	2,398,000
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Expected contributions	$ 0