Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Salaries, Wages and Officers' Compensation [Abstract]
Stock-based compensation expense	$ 1,500	$ 1,200	$ 1,200
Defined Benefit Plan Disclosure [Line Items]
Pension cost	4,400	4,200	7,800
Regulatory liabilities	26,802	23,375
Defined contribution plan matching contributions expense	2,200	2,000	2,000
Retiree health care and life insurance benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 3,400	$ 4,300	$ 1,700