Pensions (Details) - Pension Plans [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Net Periodic Pension Cost [Abstract]
Service cost	$ 255	$ 292	$ 509	$ 583
Interest cost	400	379	800	758
Expected return on plan assets	(559)	(558)	(1,117)	(1,115)
Amortization of actuarial loss	140	176	280	352
Amortization of prior service cost	(3)	(3)	(6)	(6)
Rate-regulated adjustment	342	289	684	578
Net periodic pension expense	$ 575	$ 575	1,150	$ 1,150
Employer Contributions [Abstract]
Estimated employer contributions in 2016					$ 2,300
Contributions made by employer			2,300
Estimated future contributions in current year			$ 0