Pension and Other Benefit Plans - Asset Allocations (Details)	Mar. 31, 2019
Defined Benefit Pension Plan | Cash
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	2.50%
Defined Benefit Pension Plan | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	30.00%
Defined Benefit Pension Plan | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	22.50%
Defined Benefit Pension Plan | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	10.00%
Defined Benefit Pension Plan | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	35.00%
Retiree Medical Plan | Cash
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	5.00%
Retiree Medical Plan | Equity funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	50.00%
Retiree Medical Plan | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	45.00%
Retiree Medical Plan | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	0.00%
Retiree Medical Plan | Other
Defined Benefit Plan Disclosure [Line Items]
Asset allocation	0.00%