Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Computation of basic and diluted earnings per share
Numerator for basic and diluted earnings per share - net income attributable to common stockholders	$ 101,763	$ 64,384	$ 154,432	$ 437,671	$ (111,523)	$ 74,043	$ 188,429	$ 312,639	$ 758,250	$ 463,595	$ 1,012,397
Denominator for basic earnings per share: weighted-average shares (in shares)									373,620	367,237	358,275
Effect of dilutive securities:
Employee stock options (in shares)									9	47	110
Non-vested restricted shares (in shares)									512	482	449
Redeemable shares (in shares)									1,096	1,235	1,393
Employee stock purchase program (in shares)									13	0	0
Dilutive potential common shares (in shares)									1,630	1,764	1,952
Denominator for diluted earnings per share: adjusted-weighted average shares (in shares)									375,250	369,001	360,227
Basic earnings per share (in USD per share)	$ 0.27	$ 0.17	$ 0.42	$ 1.18	$ (0.31)	$ 0.20	$ 0.51	$ 0.86	$ 2.03	$ 1.26	$ 2.83
Diluted earnings per share (in USD per share)	$ 0.27	$ 0.17	$ 0.41	$ 1.17	$ (0.31)	$ 0.20	$ 0.51	$ 0.86	$ 2.02	$ 1.26	$ 2.81