Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 254	$ 270
Interest cost	379	398
Expected return on plan assets	(698)	(599)
Amortization of actuarial loss	101	123
Amortization of prior service credit	(3)	(3)
Rate-regulated adjustment	542	386
Net periodic pension expense	575	$ 575
Employer Contributions [Abstract]
Estimated employer contributions in 2018			$ 2,300
Contributions made by employer	575
Estimated future contributions in current year	$ 1,725