Regulatory Assets and Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Regulatory Assets [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	$ 503,131	$ 354,671
Regulatory Assets [Member] | Pension [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	139,050	58,474
Regulatory Assets [Member] | Employee related benefits [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	19,080	17,700
Regulatory Assets [Member] | Distribution infrastructure projects
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	9,407	12,543
Regulatory assets, remaining amortization period	P3Y
Regulatory Assets [Member] | Environmental clean-up [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	13,741	14,924
Regulatory Assets [Member] | Supply costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	29,200	17,875
Regulatory assets, remaining amortization period	P1Y
Regulatory Assets [Member] | Income taxes [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	263,764	201,808
Regulatory Assets [Member] | Deferred financing costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	12,151	13,919
Regulatory Assets [Member] | State And Local Taxes And Fees [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	5,319	6,582
Regulatory Assets [Member] | Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory assets	11,419	10,846
Regulatory Liabilities [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	418,397	394,459
Regulatory Liabilities [Member] | Removal cost
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	351,676	336,613
Regulatory Liabilities [Member] | Gas storage sales
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	10,410	10,831
Regulatory liability, remaining amortization period	P25Y
Regulatory Liabilities [Member] | Supply costs [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	14,569	11,493
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Deferred revenue [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	36,592	33,400
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Environmental clean-up [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	2,501	1,194
Regulatory Liabilities [Member] | State And Local Taxes And Fees [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	511	551
Regulatory liability, remaining amortization period	P1Y
Regulatory Liabilities [Member] | Other [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory liabilities	$ 2,138	$ 377