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Fair Value Measurements Fair Value Measurements - Changes in Fair Value of Risk Management Assets and Liabilities (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Total net gains (losses) realized/unrealized:    
Net derivative beginning balance $ (32,979,000) $ (41,386,000)
Included in earnings 0 0
Included in OCI 88,000 (452,000)
Deferred as a regulatory asset or liability (37,543,000) (4,009,000)
Settlements 15,146,000 14,809,000
Transfers into Level 3 from Level 2 1,900,000 (6,256,000)
Transfers from Level 3 into Level 2 5,982,000 4,315,000
Net derivative ending balance (47,406,000) (32,979,000)
Net unrealized gains included in earnings related to instruments still held at end of period 0 $ 0
Significant level 1 transfers $ 0