Earnings Per Share (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Computation of basic and diluted earnings per share
Numerator for basic and diluted earnings per share - net income (loss) attributable to common stockholders	$ 190,799,000	$ 50,022,000
Denominator for basic earnings per share - weighted average shares	336,754	289,606
Effect of dilutive securities:
Employee stock options	200,000	148,000
Non-vested restricted shares	416,000	540,000
Convertible senior unsecured notes	442,000	623,000
Dilutive potential common shares	1,058,000	1,311,000
Denominator for diluted earnings per share - adjusted weighted average shares	337,812	290,917
Basic earnings per share	$ 0.57	$ 0.17
Diluted earnings per share	$ 0.56	$ 0.17