Fair Value Measurements and Derivative Instruments (Cash Flows From Derivative Instruments) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value Disclosures [Abstract]
Cash paid on settlement of derivative financial instruments	$ (63,777)	$ (110,637)	$ (2,148)
Cash paid on settlement of derivative financial instruments	(553)	(323,839)	(180,745)
Cash received (paid) on settlement of derivative financial instruments	$ 63,224	$ (213,202)	$ (178,597)