Fair Value (Details) - USD ($)	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	May. 02, 2013	Nov. 02, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	$ 542,740,000		$ 550,000,000
Amount of difference between fair value and carrying value	(111,518,000)		(51,375,000)
Senior Notes Due 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	292,740,000	$ 292,740,000	300,000,000		$ 300,000,000.0
Amount of difference between fair value and carrying value	(44,643,000)		(15,750,000)
Senior Notes Due 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of senior notes	250,000,000		250,000,000	$ 250,000,000
Amount of difference between fair value and carrying value	(66,875,000)		(35,625,000)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	431,222,000		498,625,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member] | Senior Notes Due 2020 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	248,097,000		284,250,000
Fair Value, Measurements, Nonrecurring [Member] | Fair Value, Inputs, Level 1 [Member] | Senior Notes Due 2021 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of senior notes	$ 183,125,000		$ 214,375,000