Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 270	$ 254	$ 810	$ 763
Interest cost	398	399	1,194	1,199
Expected return on plan assets	(599)	(558)	(1,796)	(1,675)
Amortization of actuarial loss	123	141	369	421
Amortization of prior service credit	(3)	(3)	(9)	(9)
Rate-regulated adjustment	386	342	1,157	1,026
Net periodic pension expense	575	$ 575	1,725	$ 1,725
Employer Contributions [Abstract]
Estimated employer contributions in 2017	2,300		2,300
Contributions made by employer			1,725
Estimated future contributions in current year	$ 575		$ 575