Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 270	$ 255	$ 540	$ 509
Interest cost	398	400	796	800
Expected return on plan assets	(598)	(559)	(1,197)	(1,117)
Amortization of actuarial loss	123	140	246	280
Amortization of prior service credit	(3)	(3)	(6)	(6)
Rate-regulated adjustment	385	342	771	684
Net periodic pension expense	$ 575	$ 575	1,150	$ 1,150
Employer Contributions [Abstract]
Estimated employer contributions in 2017					$ 2,300
Contributions made by employer			1,150
Estimated future contributions in current year			$ 1,150