Fair Value Measurements Fair Value Measurements - Changes in Fair Value of Risk Management Assets and Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Total net gains (losses) realized/unrealized:
Net derivative beginning balance	$ (47,406,000)	$ (32,979,000)
Included in earnings	0	0
Included in OCI	3,000	88,000
Deferred as a regulatory asset or liability	(13,643,000)	(37,543,000)
Settlements	5,834,000	15,146,000
Transfers into Level 3 from Level 2	(10,026,000)	1,900,000
Transfers from Level 3 into Level 2	46,982,000	5,982,000
Net derivative ending balance	(18,256,000)	(47,406,000)
Net unrealized gains included in earnings related to instruments still held at end of period	0	$ 0
Significant level 1 transfers	$ 0