Pensions (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Contributions by employer	$ 1,593
Estimated employer contributions	1,593
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	297	263
Interest cost	320	322
Expected return on plan assets	(411)	(360)
Amortization of actuarial loss	174	160
Amortization of prior service cost	3	4
Rate-regulated adjustment	15	9
Net periodic pension expense	$ 398	$ 398