Derivatives - Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative Assets
Gross Amounts	$ 1,079	$ 5,582
Gross Amounts Offset	(1,079)	(4,272)
Amounts Net of Offset	0	1,310
Financial Collateral	0	0
Net Amount	0	1,310
Derivative Liabilities
Gross Amounts	45,004	18,467
Gross Amounts Offset	(1,079)	(4,272)
Amounts Net of Offset	43,925	14,195
Financial Collateral	(43,925)	(14,195)
Net Amount	0	0
Current Assets
Derivative Assets
Gross Amounts	1,079	5,582
Gross Amounts Offset	(1,079)	(4,272)
Amounts Net of Offset	0	1,310
Financial Collateral	0	0
Net Amount	0	1,310
Noncurrent Assets
Derivative Assets
Gross Amounts	0	0
Gross Amounts Offset	0	0
Amounts Net of Offset	0	0
Financial Collateral	0	0
Net Amount	0	0
Current Liabilities
Derivative Liabilities
Gross Amounts	36,362	14,899
Gross Amounts Offset	(1,079)	(4,272)
Amounts Net of Offset	35,283	10,627
Financial Collateral	(35,283)	(10,627)
Net Amount	0	0
Noncurrent Liabilities
Derivative Liabilities
Gross Amounts	8,642	3,568
Gross Amounts Offset	0	0
Amounts Net of Offset	8,642	3,568
Financial Collateral	(8,642)	(3,568)
Net Amount	$ 0	$ 0