Investments - Narrative (Details) - Fixed income securities - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
ARIZONA PUBLIC SERVICE COMPANY
Schedule of Held-to-maturity Securities [Line Items]
Coal reclamation escrow account	$ 30,000,000
Proceeds from sale of securities	4,000,000	$ 0
4CA
Schedule of Held-to-maturity Securities [Line Items]
Coal reclamation escrow account	$ 2,000,000