Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Components of Net Periodic Pension Cost [Abstract]
Service cost	$ 212	$ 254	$ 424	$ 508
Interest cost	411	379	822	758
Expected return on plan assets	(683)	(698)	(1,366)	(1,396)
Amortization of actuarial loss	105	102	210	203
Amortization of prior service cost	(3)	(4)	(6)	(7)
Rate-regulated adjustment	533	542	1,066	1,084
Net periodic pension expense	575	$ 575	1,150	$ 1,150
Employer Contributions [Abstract]
Estimated employer contributions in 2019					$ 2,300
Contributions made by employer			1,150
Estimated future contributions in current year	$ 1,150		$ 1,150