Operations and Significant Accounting Policies - Concentration of Credit Risk (Details) - Customer Concentration Risk [Member] - Large Power Customers [Member] $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Customers	Dec. 31, 2014 USD ($)	Dec. 31, 2013
Concentration Risk [Line Items]
Number of Customers	9
Accounts Receivable | $	$ 9.2	$ 14.7
Largest Customer [Member]
Concentration Risk [Line Items]
Number of Customers	1
Largest Customer [Member] | Consolidated Operating Revenue [Member]
Concentration Risk [Line Items]
Major Customer, Percent of Consolidated Revenue	7.70%	11.90%	12.00%