Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Taxes and Interest Paid:
Income taxes paid	$ 13,615	$ 10,916	$ 14,817
Interest paid	22,762	22,305	21,822
Non-Cash Transactions:
Accrued payables for investment in utility plant	20,131	17,236	20,655
Property installed by developers and conveyed	2,082	5,395	3,284
GSWC
Taxes and Interest Paid:
Income taxes paid	4,822	8,437	1,541
Interest paid	22,282	22,078	21,797
Non-Cash Transactions:
Accrued payables for investment in utility plant	20,128	17,207	20,655
Property installed by developers and conveyed	$ 2,082	$ 5,395	$ 3,284