Long-Term Debt (Covenants) (Details) - USD ($)		12 Months Ended
	Apr. 20, 2020	Mar. 31, 2021	Jul. 14, 2020	Jun. 30, 2020	Apr. 29, 2020	Jul. 23, 2019	Mar. 16, 2018	Jan. 31, 2018	Mar. 31, 2015
2015 Facility
Long-Term Debt
Original loan amount									$ 758,000,000
The amount of restricted cash released							$ 1,600,000	$ 1,600,000
Maximum consolidated net debt to consolidated total capitalization ratio (as a percent)		60.00%
Minimum stockholder's equity balance		$ 400
Current assets and long-term restricted cash divided by current liabilities ratio (as a percent)		100.00%
Amendment to the 2015 Debt Facility
Ownership percentage of common shares by any shareholder other than certain entities or directors or officers that permits lenders to accelerate indebtedness (as a percent)						33.30%
2015 Facility | LIBOR
Long-Term Debt
Margin added to LIBOR for interest rate on loan facility (as a percent)		2.40%
Refinancing Of Commercial Tranche Of 2015 Facility
Long-Term Debt
Additional basis points to decrease the margin	0.10%				0.10%
Security leverage ratio	40.00%				40.00%
Basis points receivable as increase or reduction for changes in Average Efficiency Ratio	0.10%				0.10%
Refinancing Of Commercial Tranche Of 2015 Facility | LIBOR
Long-Term Debt
Margin added to LIBOR for interest rate on loan facility (as a percent)	2.50%
Refinancing Of Commercial Tranche Of 2015 Facility | Maximum
Long-Term Debt
Security leverage ratio	60.00%				60.00%
Refinancing Of Commercial Tranche Of 2015 Facility | Minimum
Long-Term Debt
Additional basis points to increase the margin	0.10%				0.10%
New senior secured term loan facility
Long-Term Debt
Original loan amount					$ 155,800,000
New senior secured revolving credit facility | Maximum
Long-Term Debt
Original loan amount					$ 25,000,000.0
2015 AR Facility
Long-Term Debt
Minimum cash per mortgaged vessel from first anniversary through thereafter		$ 1,000,000.0	$ 1,000,000.0	$ 2,200,000
Minimum liquidity covenant		$ 27,500,000
Percentage of increase in security value ratio		145.00%