Interest on Borrowings (Interest Rates and Company Borrowings) (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Interest and debt expense:
Weighted average interest rate during the year	1.40%	3.31%	3.39%
Interest rate at year end	1.40%	2.86%	3.60%
Maximum amount outstanding during the year	$ 1,175,000	$ 1,086,000	$ 959,400
Average amount outstanding during the year	1,088,293	1,002,081	699,415
Facility fees	$ 261	$ 193	$ 374