Fair Value of Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Fair Value of Financial Assets and Liabilities
Risk premium (as a percent)	1.83%
Advances for construction	$ 189,642	$ 186,342
Level 1
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	0	0
Advances for construction	0	0
Total	0	0
Level 2
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	862,181	849,551
Advances for construction	79,834	77,204
Total	942,015	926,755
Level 3
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	0	0
Advances for construction	0	0
Total	0	0
Cost
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	813,005	814,938
Advances for construction	189,642	186,342
Total	1,002,647	1,001,280
Total
Fair Value of Financial Assets and Liabilities
Long-term debt, including current maturities, net	862,181	849,551
Advances for construction	79,834	77,204
Total	$ 942,015	$ 926,755