Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Pension cost	$ 7,800,000	$ 7,300,000	$ 5,700,000
Regulatory liabilities	19,252,000	17,395,000
Defined contribution plan matching contributions expense	2,000,000	2,500,000	2,400,000
Salaries, Wages and Officers' Compensation [Abstract]
Stock-based compensation expense	1,200,000	1,600,000	1,300,000
Retiree health care and life insurance benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	$ 1,700,000		$ 1,100,000