Dividends on common stock: (Details) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended
	Dec. 03, 2021	Nov. 02, 2021	Sep. 30, 2021	Sep. 30, 2020
Dividends on common stock and return of capital program
Dividends paid			$ 110,736	$ 94,952
Subsequent Event
Dividends on common stock and return of capital program
Quarterly dividend payment approved (per share)		$ 0.83
Estimated
Dividends on common stock and return of capital program
Dividends paid	$ 38,400