Risk Management and Hedging Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Physical purchase and sale of gas and electricity at fixed prices	$ 0	$ 0
Pre-tax gain on cash flow hedge from AOCL to be reclassified during next 12 months	600
No swaps outstanding, interest rate fair value derivatives	0	$ 0
Interest Rate Swap [Member]
Derivative [Line Items]
Pre-tax loss on cash flow hedges remaining in AOCL	17,100
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative [Line Items]
Interest rate contracts, amount of gain reclassified from AOCL into income	$ 2,169