Financial Instruments and Fair Value Disclosures (Swaps) (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017
Interest rate swaps
Derivative Instruments
Nominal value	$ 483,697,639	$ 522,832,792
1.933% interest rate swap due on March 2022 | Citibank N.A.
Derivative Instruments
Fixed interest rate (as a percent)	1.933%
Nominal value	$ 200,000,000	200,000,000
Final settlement amount	$ 200,000,000
2.002% interest rate swap due on March 2022 | ING Bank N. V. Member
Derivative Instruments
Fixed interest rate (as a percent)	2.002%
Nominal value	$ 50,000,000	50,000,000
Final settlement amount	$ 50,000,000
1.428% interest rate swap due on March 2022 | CBA
Derivative Instruments
Fixed interest rate (as a percent)	1.428%
Nominal value	$ 60,025,000	71,250,000
Quarterly reduction of notional amount	2,800,000
Final settlement amount	$ 17,900,000
1.380% interest rate swap due on March 2022 | Citibank N.A.
Derivative Instruments
Fixed interest rate (as a percent)	1.38%
Nominal value	$ 90,037,500	106,875,000
Quarterly reduction of notional amount	4,200,000
Final settlement amount	$ 26,900,000
1.213% interest rate swap due March 2022 | Citibank N.A.
Derivative Instruments
Fixed interest rate (as a percent)	1.213%
Nominal value	$ 59,276,849	67,124,650
Quarterly reduction of notional amount	2,000,000
Final settlement amount	$ 29,900,000
1.161% interest rate swap due March 2022 | Citibank N.A.
Derivative Instruments
Fixed interest rate (as a percent)	1.161%
Nominal value	$ 24,358,290	$ 27,583,142
Quarterly reduction of notional amount	800,000
Final settlement amount	$ 12,300,000