Revision to Prior Year Information (Consolidated Statements of Equity) (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive income (loss):
Net income (loss)	$ (85,465)	$ 84,897	$ 723,984	$ (328,199)	$ 348,194	$ (136,506)	$ 214,381	$ 81,934	$ 395,785	$ 395,217	$ 508,003	$ 779,668
As Previously Reported
Comprehensive income (loss):
Net income (loss)			694,806	(326,131)					368,675		512,898	785,803
Revision
Comprehensive income (loss):
Net income (loss)			29,178	(2,068)					27,110		(4,895)	(6,135)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		1,437,413	3,306,948	3,635,147				3,557,144	3,635,147	3,635,147	3,557,144	3,525,476
Comprehensive income (loss):
Net income (loss)				(328,199)					395,785	395,217	508,003	779,668
Ending Balance	1,437,266		1,437,413	3,306,948	3,635,147				1,437,413	1,437,266	3,635,147	3,557,144
Retained Earnings | As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		1,438,695	3,337,408	3,663,539				3,580,641	3,663,539	3,663,539	3,580,641	3,542,838
Comprehensive income (loss):
Net income (loss)				(326,131)					368,675		512,898	785,803
Ending Balance			1,438,695	3,337,408	3,663,539				1,438,695		3,663,539	3,580,641
Retained Earnings | Revision
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		(1,282)	(30,460)	(28,392)				(23,497)	(28,392)	(28,392)	(23,497)	(17,362)
Comprehensive income (loss):
Net income (loss)				(2,068)					27,110		(4,895)	(6,135)
Ending Balance			(1,282)	(30,460)	(28,392)				(1,282)		(28,392)	(23,497)
Total Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		2,666,992	4,274,493	4,482,425				4,530,488	4,482,425	4,482,425	4,530,488	4,388,859
Comprehensive income (loss):
Net income (loss)				(328,199)					395,785	395,217	508,003	779,668
Total comprehensive income (loss)				(212,872)					571,996	401,387	394,117	901,713
Ending Balance	$ 2,496,803		2,666,992	4,274,493	4,482,425				2,666,992	2,496,803	4,482,425	4,530,488
Total Equity | As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		2,668,274	4,304,953	4,510,817				4,553,985	4,510,817	4,510,817	4,553,985	4,406,221
Comprehensive income (loss):
Net income (loss)				(326,131)					368,675		512,898	785,803
Total comprehensive income (loss)				(210,804)					544,886		399,012	907,848
Ending Balance			2,668,274	4,304,953	4,510,817				2,668,274		4,510,817	4,553,985
Total Equity | Revision
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance		$ (1,282)	(30,460)	(28,392)				$ (23,497)	(28,392)	$ (28,392)	(23,497)	(17,362)
Comprehensive income (loss):
Net income (loss)				(2,068)					27,110		(4,895)	(6,135)
Total comprehensive income (loss)				(2,068)					27,110		(4,895)	(6,135)
Ending Balance			$ (1,282)	$ (30,460)	$ (28,392)				$ (1,282)		$ (28,392)	$ (23,497)