Significant Accounting Policies - Earnings per share (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant Accounting Policies
Net (loss) income available for stockholders	$ (212,192)	$ (87,111)	$ 175,402
Less: Adjustments for losses attributable to participating securities		(334)	(1,839)
Net (loss) income used in calculating per share amounts	$ (212,192)	$ (87,445)	$ 173,563
Weighted average shares outstanding (including participating securities)	215,063	214,730	215,198
Adjustment for participating securities	(2,571)	(2,509)	(2,453)
Shares used in calculating basic and diluted earnings (loss) per share	212,492	212,221	212,745