Dividends on common stock: (Details) - USD ($) $ / shares in Units, $ in Thousands			6 Months Ended
	Sep. 04, 2020	Aug. 05, 2020	Jun. 30, 2020	Jun. 30, 2019
Dividends on common stock and return of capital program
Quarterly dividend payment approved (per share)		$ 0.705
Dividends paid			$ 62,295	$ 54,306
Estimated
Dividends on common stock and return of capital program
Dividends paid	$ 32,200