Other (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Capitalization of project feasibility costs previously expensed	$ 3,500,000		$ 11,100,000
Operation and maintenance	271,603,000	264,631,000	299,734,000
Total property, plant and equipment, net	6,387,538,000	5,777,392,000
Regulatory assets	316,900,000	294,100,000
Other expense, net	(37,208,000)	(32,606,000)	(21,230,000)
Eminence Storage Field [Member]
Property, Plant and Equipment [Line Items]
Operation and maintenance	800,000	4,300,000	2,500,000
Total property, plant and equipment, net		(92,000,000)
Regulatory assets		92,000,000
Other expense, net	2,900,000	(11,500,000)
Insurance recoveries		$ 16,100,000