Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	$ 64,384	$ 74,043	$ 656,487	$ 575,118
Denominator for basic earnings per share - weighted average shares (in shares)	373,023,000	369,089,000	372,052,000	366,096,000
Effect of dilutive securities:
Employee stock options (in shares)	6,000	40,000	12,000	53,000
Non-vested restricted shares (in shares)	348,000	515,000	464,000	464,000
Redeemable shares (in shares)	1,096,000	1,096,000	1,096,000	1,281,000
Employee stock purchase program (in shares)	14,000	0	14,000	0
Dilutive potential common shares (in shares)	1,464,000	1,651,000	1,586,000	1,798,000
Denominator for diluted earnings per share - adjusted weighted average shares (in shares)	374,487,000	370,740,000	373,638,000	367,894,000
Basic earnings per share (USD per share)	$ 0.17	$ 0.20	$ 1.76	$ 1.57
Diluted earnings per share (USD per share)	$ 0.17	$ 0.20	$ 1.76	$ 1.56