Commitments and Contingencies - Narrative - Environmental Matters (Details)			6 Months Ended	12 Months Ended
	May 09, 2017	Jun. 30, 2000 respondent water_company	Jun. 30, 2018 USD ($) LegalMatter	Dec. 31, 2002 USD ($) a	Dec. 31, 2017 USD ($)	Oct. 31, 2001 USD ($)
Site Contingency [Line Items]
Accrued environmental costs			$ 333,700,000		$ 341,400,000
Guarantee obligations			$ 120,000,000
Various Environmental Matters
Site Contingency [Line Items]
Number of environmental remediation matters (over) | LegalMatter			40
Environmental remediation term			15 years
Accrued environmental costs			$ 333,700,000
Environmental obligation funding percent			99.00%
Various Environmental Matters | Minimum
Site Contingency [Line Items]
Estimated environmental costs			$ 333,700,000
Various Environmental Matters | Maximum
Site Contingency [Line Items]
Estimated environmental costs			493,100,000
Various Environmental Matters | Sacramento, California
Site Contingency [Line Items]
Accrued environmental costs			205,400,000
Various Environmental Matters | Sacramento, California | Environmental Protection Agency
Site Contingency [Line Items]
Non-contaminated land | a				2,600
Various Environmental Matters | Sacramento, California | Minimum
Site Contingency [Line Items]
Estimated environmental costs			205,400,000
Various Environmental Matters | Sacramento, California | Maximum
Site Contingency [Line Items]
Estimated environmental costs			325,400,000
Guarantee obligations				$ 75,000,000
Various Environmental Matters | Sacramento, California | As Reported
Site Contingency [Line Items]
Guarantee obligations				$ 20,000,000
Various Environmental Matters | Baldwin Park Operable Unit
Site Contingency [Line Items]
Accrued environmental costs			$ 110,400,000
Guarantee obligations						$ 25,000,000
Management agreement terms	10 years
Various Environmental Matters | Baldwin Park Operable Unit | Aerojet Rocketdyne, Inc.
Site Contingency [Line Items]
Percentage of responsibility in all project costs			74.00%
Various Environmental Matters | Baldwin Park Operable Unit | Environmental Protection Agency
Site Contingency [Line Items]
Number of respondents | respondent		7
Number of water companies | water_company		5
Various Environmental Matters | Baldwin Park Operable Unit | Minimum
Site Contingency [Line Items]
Estimated environmental costs			$ 110,400,000
Various Environmental Matters | Baldwin Park Operable Unit | Maximum
Site Contingency [Line Items]
Estimated environmental costs			$ 140,800,000