Risk Management and Hedging Activities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Pre-tax gain on cash flow hedges remaining in AOCL	$ 17,200
Pre-tax gain on cash flow hedge to be reclassified within twelve months from AOCL to interest expense	600
Physical purchase and sale of gas and electricity at fixed prices	$ 0	$ 0
Number of interest rate derivatives held	0
Interest Expense [Member]
Derivative [Line Items]
Amount of gain reclassified from AOCL	$ 2,324