Debt (Details) (USD $)	3 Months Ended	6 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 item	Mar. 31, 2013
Debt
Long-term debt	$ 704,290,000	$ 704,290,000	$ 708,790,000
Less portion classified as current	(60,500,000)	(60,500,000)	(65,000,000)
Long-term debt, less current portion	643,790,000	643,790,000	643,790,000
Actual fixed charge coverage ratio	2.10	2.10
Borrowings outstanding	60,500,000	60,500,000	65,000,000
Number of independent assets or operations other than investments in subsidiaries	0	0
Restricted assets	0	0
Minimum
Debt
Fixed charge coverage ratio required to incur additional debt obligations		2
Senior Notes due 2018
Debt
Long-term debt	643,790,000	643,790,000	643,790,000
Interest rate (as a percent)	8.875%	8.875%
Principal payments required prior to maturity	0	0
Estimated fair value of debt	674,700,000	674,700,000	669,500,000
Number of preceding quarters for which fixed charge coverage ratio is measured		4
Amount permitted to be distributed by the indenture under the limitation	252,700,000	252,700,000
Minimum fixed charge coverage ratio required to make restricted payments	1.75
Amount included in the general basket of restricted payments	75,000,000
Actual fixed charge coverage ratio	2.10	2.10
Revolving credit facility
Debt
Long-term debt	60,500,000	60,500,000	65,000,000
Maximum borrowing capacity	400,000,000	400,000,000
Borrowing base collateral	388,500,000	388,500,000
Borrowings outstanding	60,500,000	60,500,000	65,000,000
Weighted average interest rate (as a percent)	3.94%	3.94%	3.69%
Letters of credit issued	$ 34,800,000	$ 34,800,000	$ 3,300,000