Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Computation of basic and diluted earnings per share
Numerator for basic and diluted earnings per share - net income (loss) attributable to common stockholders	$ 154,432	$ 188,429	$ 592,103	$ 501,068
Denominator for basic earnings per share - weighted average shares (in shares)	371,640,000	366,524,000	371,552,000	364,551,000
Effect of dilutive securities:
Employee stock options (in shares)	14,000	50,000	15,000	60,000
Non-vested restricted shares (in shares)	325,000	479,000	523,000	438,000
Redeemable shares (in shares)	1,096,000	1,096,000	1,096,000	1,374,000
Dilutive potential common shares (in shares)	1,435,000	1,625,000	1,634,000	1,872,000
Denominator for diluted earnings per share - adjusted weighted average shares (in shares)	373,075,000	368,149,000	373,186,000	366,423,000
Basic earnings per share (USD per share)	$ 0.42	$ 0.51	$ 1.59	$ 1.37
Diluted earnings per share (USD per share)	$ 0.41	$ 0.51	$ 1.59	$ 1.37