Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Apr. 09, 2019	Mar. 31, 2019
Long-Term Debt
Face amount	$ 2,794,465		$ 2,794,465			$ 2,172,789
Face amount, current portion	649		649			648
Face amount, long-term	2,793,816		2,793,816			2,172,141
LONG-TERM DEBT, debt issuance costs	(20,581)		(20,581)			(12,008)
Debt issuance costs, current, net	0		0			0
Debt issuance costs, noncurrent, net	(20,581)		(20,581)			(12,008)
Book value	2,773,884		2,773,884			2,160,781
Book value, current	649		649			648
LONG-TERM DEBT, net of debt issuance costs and current maturities	2,773,235		2,773,235			2,160,133
Amortization of debt issuance costs	1,000	$ 1,200	1,800	$ 2,500
Expected Future Amortization of Debt Issuance Costs
2020 (six months)	2,571		2,571
2021	3,867		3,867
2022	3,854		3,854
2023	3,854		3,854
2024	3,230		3,230
Thereafter	3,205		3,205
Total	$ 20,581		$ 20,581
7.50% Senior Notes due 2023
Long-Term Debt
Fixed interest rate	7.50%		7.50%
Face amount	$ 607,323		$ 607,323			607,323
LONG-TERM DEBT, debt issuance costs	(6,159)		(6,159)			(6,916)
Book value	$ 601,164		$ 601,164			600,407
6.125% Senior Notes due 2025
Long-Term Debt
Fixed interest rate	6.125%		6.125%
Face amount	$ 389,135		$ 389,135			389,135
LONG-TERM DEBT, debt issuance costs	(4,665)		(4,665)			(5,092)
Book value	$ 384,470		$ 384,470			384,043
7.50% Senior Notes due 2026
Long-Term Debt
Fixed interest rate	7.50%		7.50%		7.50%
Face amount	$ 450,000		$ 450,000			0
LONG-TERM DEBT, debt issuance costs	(7,452)		(7,452)			0
Book value	442,548		442,548			0
Term Credit Agreement
Long-Term Debt
Face amount	250,000		250,000
LONG-TERM DEBT, debt issuance costs	(2,305)		(2,305)
Book value	247,695		247,695
Other long-term debt
Long-Term Debt
Face amount	5,007		5,007			5,331
LONG-TERM DEBT, debt issuance costs	0		0			0
Book value	5,007		5,007			5,331
Expansion Capital Facility | Revolving Credit Facility
Long-Term Debt
Face amount	450,000		450,000			275,000
LONG-TERM DEBT, debt issuance costs	0		0			0
Book value	450,000		450,000			275,000
Working Capital Facility | Revolving Credit Facility
Long-Term Debt
Face amount	643,000		643,000			896,000
LONG-TERM DEBT, debt issuance costs	0		0			0
Book value	$ 643,000		$ 643,000			$ 896,000