Long-Term Debt (Details) $ in Thousands	1 Months Ended	3 Months Ended			6 Months Ended
	Oct. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Mar. 31, 2016 USD ($)	Jul. 09, 2014 USD ($)	Oct. 16, 2013 USD ($)	Jun. 19, 2012 USD ($)
Long-Term Debt
Face amount		$ 3,084,536			$ 3,084,536		$ 2,936,244
Face amount, current portion		8,046			8,046		7,907
Face amount, long-term		3,076,490			3,076,490		2,928,337
Unamortized debt issuance costs		(13,482)			(13,482)		(15,500)
Book value		3,071,054			3,071,054		2,920,744
Book value, current		8,046			8,046		7,907
LONG-TERM DEBT, net of debt issuance costs and current maturities		3,063,008			3,063,008		2,912,837
Amortization of debt issuance costs		1,000		$ 800	1,800	$ 1,600
Repurchases of senior notes					15,129	$ 0
Expected Future Amortization of Debt Issuance Costs [Abstract]
2017 (six months)		1,619			1,619
2018		3,229			3,229
2019		3,226			3,226
2020		2,236			2,236
2021		1,827			1,827
Thereafter		1,345			1,345
Total		13,482			13,482
Revolving Credit Facility
Long-Term Debt
Long-term line of credit		$ 2,000,000			$ 2,000,000
Interest rate		2.83%			2.83%
Revolving Credit Facility
Long-Term Debt
Maximum borrowing capacity		$ 2,484,000			$ 2,484,000
Increased borrowing capacity with the exercise of accordion feature		150,000			$ 150,000
Actual leverage ratio					4.15
Actual interest coverage ratio					4.60
Revolving Credit Facility | Minimum
Long-Term Debt
Commitments fees charged on unused capacity					0.38%
Interest coverage ratio					2.75
Revolving Credit Facility | Maximum
Long-Term Debt
Commitments fees charged on unused capacity					0.50%
Leverage ratio					4.75
Revolving Credit Facility | Alternate base rate | Minimum
Long-Term Debt
Interest rate margin added to variable rate base					0.50%
Revolving Credit Facility | Alternate base rate | Maximum
Long-Term Debt
Interest rate margin added to variable rate base					1.75%
Revolving Credit Facility | LIBOR option
Long-Term Debt
Interest rate margin added to variable rate base					2.25%
Reference rate					0.53%
Revolving Credit Facility | LIBOR option | Minimum
Long-Term Debt
Interest rate margin added to variable rate base					1.50%
Revolving Credit Facility | LIBOR option | Maximum
Long-Term Debt
Interest rate margin added to variable rate base					2.75%
Revolving Credit Facility | Prime Rate
Long-Term Debt
Interest rate margin added to variable rate base					1.25%
Reference rate					3.50%
Revolving Credit Facility | Expansion Capital Facility
Long-Term Debt
Face amount		1,312,000			$ 1,312,000		1,229,500
Unamortized debt issuance costs		0			0		0
Book value		1,312,000			1,312,000		1,229,500
Long-term line of credit		1,446,000			1,446,000
Revolving Credit Facility | Working Capital Facility
Long-Term Debt
Face amount		710,500			710,500		618,500
Unamortized debt issuance costs		0			0		0
Book value		710,500			710,500		618,500
Maximum borrowing capacity		1,038,000			1,038,000
Outstanding letters of credit		$ 75,300			$ 75,300
Revolving Credit Facility | Letters of credit
Long-Term Debt
Fixed interest rate		2.25%			2.25%
5.125% Senior Notes due 2019
Long-Term Debt
Face amount		$ 383,467			$ 383,467		388,467
Fixed interest rate		5.125%			5.125%			5.125%
Unamortized debt issuance costs		$ (3,871)			$ (3,871)		(4,681)
Book value		379,596			379,596		383,786
Debt issued								$ 400,000
Repurchase amount		5,000			5,000
Repurchases of senior notes		3,100
Gain on early extinguishment of debt		1,800
Write off of debt issuance cost		100
6.875% Senior Notes due 2021
Long-Term Debt
Face amount		$ 369,063			$ 369,063		388,289
Fixed interest rate		6.875%			6.875%				6.875%
Unamortized debt issuance costs		$ (6,433)			$ (6,433)		(7,545)
Book value		362,630			362,630		380,744
Debt issued									$ 450,000
Repurchase amount			$ 19,200
Repurchases of senior notes (excluding payments of accrued interest)			12,000
Gain on early extinguishment of debt			6,800
Write off of debt issuance cost			$ 400
6.650% Senior Notes due 2022
Long-Term Debt
Face amount		$ 250,000			$ 250,000		250,000
Fixed interest rate		6.65%			6.65%					6.65%
Unamortized debt issuance costs		$ (3,055)			$ (3,055)		(3,166)
Book value		246,945			246,945		246,834
Debt issued										$ 250,000
Repayments in semi-annual installments					25,000
7.50% Senior Notes due 2023 | Subsequent Event
Long-Term Debt
Fixed interest rate	7.50%
Debt issued	$ 700,000
Proceeds from unsecured notes payable	$ 688,000
Other long-term debt
Long-Term Debt
Face amount		59,506			59,506		61,488
Unamortized debt issuance costs		(123)			(123)		(108)
Book value		$ 59,383			$ 59,383		$ 61,380
Other long-term debt | Minimum
Long-Term Debt
Fixed interest rate		1.17%			1.17%
Other long-term debt | Maximum
Long-Term Debt
Fixed interest rate		7.08%			7.08%