Stockholders' equity - Preferred stock - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	2 Months Ended	3 Months Ended		9 Months Ended
	Feb. 24, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Stockholders Equity Note
Stockholders equity		$ 5,912,085	$ 5,908,448	$ 5,912,085	$ 5,908,448	$ 5,780,165	$ 6,016,779	$ 5,719,834	$ 5,435,057
Preferred stock, shares authorized		30,000,000		30,000,000			30,000,000
Dividends, Preferred Stock, Cash		$ 352	$ 930	$ 1,405	$ 2,792
Preferred stock, shares outstanding		885,726	2,006,391	885,726	2,006,391		2,006,391		2,006,391
Preferred Stock
Stockholders Equity Note
Stockholders equity		$ 22,143	$ 50,160	$ 22,143	$ 50,160	$ 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