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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, 2015
Dec. 31, 2014
Total net gains (losses) realized/unrealized:    
Net derivative beginning balance $ (41,386,000) $ (49,165,000)
Included in earnings 0 102,000
Included in OCI (452,000) (239,000)
Deferred as a regulatory asset or liability (4,009,000) (482,000)
Settlements 14,809,000 12,080,000
Transfers into Level 3 from Level 2 (6,256,000) (2,090,000)
Transfers from Level 3 into Level 2 4,315,000 (1,592,000)
Net derivative ending balance (32,979,000) (41,386,000)
Net unrealized gains included in earnings related to instruments still held at end of period 0 $ 0
Significant level 1 transfers $ 0