Fair Value Measurements and Derivative Instruments (Recurring) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Derivative financial instruments	$ 215,634	$ 0
Liabilities:
Derivative financial instruments	11,210	346,887
Fair Value, Measurements, Recurring | Total Fair Value
Assets:
Derivative financial instruments	320,385	19,397
Investments	3,340	3,576
Total Assets	323,725	22,973
Liabilities:
Derivative financial instruments	115,961	373,497
Total Liabilities	115,961	373,497
Fair Value, Measurements, Recurring | Level 1
Assets:
Derivative financial instruments	0	0
Investments	3,340	3,576
Total Assets	3,340	3,576
Liabilities:
Derivative financial instruments	0	0
Total Liabilities	0	0
Fair Value, Measurements, Recurring | Level 2
Assets:
Derivative financial instruments	320,385	19,397
Investments	0	0
Total Assets	320,385	19,397
Liabilities:
Derivative financial instruments	115,961	373,497
Total Liabilities	115,961	373,497
Fair Value, Measurements, Recurring | Level 3
Assets:
Derivative financial instruments	0	0
Investments	0	0
Total Assets	0	0
Liabilities:
Derivative financial instruments	0	0
Total Liabilities	$ 0	$ 0