Stockholders' equity - Preferred stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	2 Months Ended	3 Months Ended
	Feb. 24, 2020	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Stockholders Equity Note
Stockholders equity		$ 5,669,605	$ 5,440,060	$ 6,016,779	$ 5,435,057
Preferred stock, shares authorized		30,000,000		30,000,000
Dividends, Preferred Stock, Cash		$ 700	$ 931
Preferred stock, shares outstanding		885,726	2,006,391	2,006,391	2,006,391
Preferred Stock
Stockholders Equity Note
Stockholders equity		$ 22,143	$ 50,160	$ 50,160	$ 50,160
Preferred Stock | 2008 Series B Preferred Stock
Stockholders Equity Note
Preferred stock redemption price per share	$ 25.00
Accrued and unpaid dividends on each share	0.1375
Dividends perferred stock per share	$ 25.1375
Preferred stock dividend rate percentage	8.25%
Dividends, Preferred Stock, Cash	$ 28,200