Financial Instruments and Fair Value Disclosures (Swaps) (Details) - USD ($)	Mar. 31, 2017	Mar. 31, 2016
Interest rate swaps
Derivative Instruments
Nominal value	$ 522,832,792	$ 564,902,000
Interest rate swaps | Royal Bank of Scotland plc (RBS)
Derivative Instruments
Final settlement amount	$ 8,100,000
5.395% interest rate swap due on Nov 2018 | RBS
Derivative Instruments
Fixed interest rate (as a percent)	5.395%
Nominal value		20,456,000
4.936% interest rate swap due on Nov 2018 | RBS
Derivative Instruments
Fixed interest rate (as a percent)	4.936%
Nominal value		7,671,000
4.772% interest rate swap due on March 2019 | RBS
Derivative Instruments
Fixed interest rate (as a percent)	4.772%
Nominal value		27,979,875
2.960% interest rate swap due on March 2019 | RBS
Derivative Instruments
Fixed interest rate (as a percent)	2.96%
Nominal value		9,420,125
4.350% interest rate swap due on July 2020 | RBS
Derivative Instruments
Fixed interest rate (as a percent)	4.35%
Nominal value		43,000,000
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	$ 71,250,000	82,550,000
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	$ 106,875,000	$ 123,825,000
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	$ 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	$ 27,583,142
Quarterly reduction of notional amount	800,000
Final settlement amount	$ 12,300,000