Regulatory Assets and Liabilities (Tables)

12 Months Ended

Dec. 31, 2016

Regulatory Assets and Liabilities Disclosure [Abstract]

Schedule of Regulatory Assets And Liabilities

	Note Reference	Remaining Amortization Period	December 31,
			2016		2015
			(in thousands)
Income taxes	13	Plant Lives	$	411,546	$	319,973
Pension	15	Undetermined	127,133		135,057
Deferred financing costs		Various	24,810		19,978
Employee related benefits	15	Undetermined	20,256		21,055
State & local taxes & fees		Various	17,838		7,724
Supply costs		1 Year	16,809		29,604
Environmental clean-up	19	Various	13,601		14,237
Distribution infrastructure projects		1 Year	3,136		6,272
Other	—	Various	17,855		14,671
Total Regulatory Assets			$	652,984	$	568,571
Removal cost	7	Various	$	386,373	$	368,467
Supply costs		1 Year	14,041		13,685
Gas storage sales		23 Years	9,569		9,990
Environmental clean-up		Various	6,383		7,089
Deferred revenue	4	1 Year	5,066		58,868
State & local taxes & fees		1 Year	1,154		1,566
Other		Various	—		36
Total Regulatory Liabilities			$	422,586	$	459,701