Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Components of Net Periodic Pension Cost [Abstract]
Service cost	$ 212	$ 254
Interest cost	411	379
Expected return on plan assets	(683)	(698)
Amortization of actuarial loss	105	101
Amortization of prior service credit	(3)	(3)
Rate-regulated adjustment	533	542
Net periodic pension expense	575	$ 575
Employer Contributions [Abstract]
Estimated employer contributions in 2019			$ 2,300
Contributions made by employer	575
Estimated future contributions in current year	$ 1,725