Debt - Narrative (Details)	3 Months Ended		9 Months Ended
	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)
Debt Instrument [Line Items]
Short-term debt	$ 25,000,000		$ 25,000,000		$ 0
Interest costs	7,600,000	$ 7,700,000	22,300,000	$ 22,900,000
Interest costs, capitalized	400,000	$ 600,000	1,500,000	$ 2,000,000
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	25,000,000		25,000,000
Credit Agreement available for future borrowings	$ 467,400,000		$ 467,400,000
Credit Agreement | LIBOR
Debt Instrument [Line Items]
Variable rate basis			LIBOR
Interest rate margin (as a percent)			1.45%
Credit Agreement | LIBOR | Minimum
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.75%
Credit Agreement | LIBOR | Maximum
Debt Instrument [Line Items]
Interest rate margin (as a percent)			1.90%
Credit Agreement | Base Rate
Debt Instrument [Line Items]
Variable rate basis			Base Rate
Interest rate margin (as a percent)			0.45%
Credit Agreement | Base Rate | Minimum
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.00%
Credit Agreement | Base Rate | Maximum
Debt Instrument [Line Items]
Interest rate margin (as a percent)			0.90%
Letters of credit
Debt Instrument [Line Items]
Commitment fee rate (as a percent)			0.225%
Letter of credit fees (as a percent)			1.45%
Required interest coverage ratio			3.50
Letters of credit | Minimum
Debt Instrument [Line Items]
Commitment fee rate (as a percent)			0.075%
Letter of credit fees (as a percent)			0.75%
Letters of credit | Maximum
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