Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Cash	$ 43,700		$ 43,700		$ 5,900
Long-term nonutility investments	5,200		5,200		$ 5,100
Net income	(5,265)	$ 11,663	16,944	$ 35,369
7.25% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Senior secured note	$ 33,500		$ 33,500
Senior notes, interest rate, percentage	7.25%		7.25%
Debt instrument, maturity date			Apr. 01, 2016
5.00% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Senior secured note	$ 400,000		$ 400,000
Senior notes, interest rate, percentage	5.00%		5.00%
Debt instrument, maturity date			May 01, 2018
3.45% Senior Secured Notes [Member]
Debt Instrument [Line Items]
Senior secured note	$ 405,000		$ 405,000
Senior notes, interest rate, percentage	3.45%		3.45%
Utility Segment [Member]
Debt Instrument [Line Items]
Net income	$ 15,300	$ 19,800	$ 45,000	$ 51,200
Maximum [Member]
Debt Instrument [Line Items]
Nonutility assets representation rate, percentage			2.00%		1.00%