Debt - Narrative (Details)	3 Months Ended		6 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2015 USD ($)
Debt Instrument [Line Items]
Short-term debt	$ 39,500,000		$ 39,500,000		$ 0
Interest costs	7,500,000	$ 7,500,000	14,700,000	$ 15,100,000
Interest costs, capitalized	500,000	$ 700,000	1,100,000	$ 1,300,000
Credit Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity	500,000,000		500,000,000
Letters of credit issued	7,600,000		7,600,000
Short-term debt	39,500,000		39,500,000
Credit Agreement available for future borrowings	$ 452,900,000		$ 452,900,000
Credit Agreement | LIBOR
Debt Instrument [Line Items]
Variable rate basis			LIBOR
Interest rate margin (as a percent)			1.25%
Credit Agreement | Base Rate
Debt Instrument [Line Items]
Variable rate basis			Base Rate
Interest rate margin (as a percent)			0.25%
Letters of credit
Debt Instrument [Line Items]
Commitment fee rate (as a percent)			0.15%
Letter of credit fees (as a percent)			1.25%
Required interest coverage ratio			3.50
Minimum | Credit Agreement | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.75%
Minimum | Credit Agreement | Base Rate
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.00%
Minimum | Letters of credit
Debt Instrument [Line Items]
Commitment fee rate (as a percent)			0.075%
Letter of credit fees (as a percent)			0.75%
Maximum | Credit Agreement | LIBOR
Debt Instrument [Line Items]
Interest rate margin (as a percent)			1.90%
Maximum | Credit Agreement | Base Rate
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.90%
Maximum | Letters of credit
Debt Instrument [Line Items]
Commitment fee rate (as a percent)			0.375%
Letter of credit fees (as a percent)			1.90%
Required debt to capital ratio (less than 55%)	0.55		0.55