Revision to Prior Year Information (Consolidated Statements of Equity) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Comprehensive income (loss):
Net income (loss)	$ 349,164	$ 723,984	$ 389,236	$ 395,785
As Previously Reported
Comprehensive income (loss):
Net income (loss)		694,806		368,675
Revision
Comprehensive income (loss):
Net income (loss)		29,178		27,110
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			1,437,266	3,635,147
Comprehensive income (loss):
Net income (loss)			389,236	395,785
Ending Balance	1,826,502	1,437,413	1,826,502	1,437,413
Retained Earnings | As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				3,663,539
Comprehensive income (loss):
Net income (loss)				368,675
Ending Balance		1,438,695		1,438,695
Retained Earnings | Revision
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				(28,392)
Comprehensive income (loss):
Net income (loss)				27,110
Ending Balance		(1,282)		(1,282)
Total Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance			2,496,803	4,482,425
Comprehensive income (loss):
Net income (loss)			389,236	395,785
Total comprehensive income (loss)			473,389	571,996
Ending Balance	$ 2,976,067	2,666,992	$ 2,976,067	2,666,992
Total Equity | As Previously Reported
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				4,510,817
Comprehensive income (loss):
Net income (loss)				368,675
Total comprehensive income (loss)				544,886
Ending Balance		2,668,274		2,668,274
Total Equity | Revision
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning Balance				(28,392)
Comprehensive income (loss):
Net income (loss)				27,110
Total comprehensive income (loss)				27,110
Ending Balance		$ (1,282)		$ (1,282)