Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 270	$ 254
Interest cost	398	400
Expected return on plan assets	(599)	(558)
Amortization of actuarial loss	123	140
Amortization of prior service credit	(3)	(3)
Rate-regulated adjustment	386	342
Net periodic pension expense	575	$ 575
Employer Contributions [Abstract]
Estimated employer contributions in 2017			$ 2,300
Contributions made by employer	575
Estimated future contributions in current year	$ 1,725