Variable Interest Entities (Details) - Sabal Trail Transmission, LLC - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Variable Interest Entity [Line Items]
Total Assets	$ 1,724	$ 916
Equity	1,596	832
Total Liabilities and Equity	1,724	916
Current assets
Variable Interest Entity [Line Items]
Total Assets	303	118
Net property, plant and equipment
Variable Interest Entity [Line Items]
Total Assets	1,363	773
Regulatory assets and deferred debits
Variable Interest Entity [Line Items]
Total Assets	58	25
Current liabilities
Variable Interest Entity [Line Items]
Current liabilities	$ 128	$ 84