Long-term Debt - Convertible Senior Notes Due 2024 (Details)	1 Months Ended		6 Months Ended
	Jul. 31, 2017 USD ($) $ / shares	Jun. 30, 2014	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2014 $ / shares
Debt Instrument [Line Items]
Principal amount			$ 338,750,000
Carrying amount			$ 301,625,000	$ 284,807,000	$ 266,724,000
2024 Convertible Notes
Debt Instrument [Line Items]
Principal amount	$ 143,750,000.00
Stated interest rate	2.875%
Effective interest rate			7.00%
If-converted value in excess of principal amount			$ 46,700,000
2019 Convertible Notes
Debt Instrument [Line Items]
Stated interest rate						2.125%
Effective interest rate			7.00%
Common Stock Class A Non-voting | 2024 Convertible Notes
Debt Instrument [Line Items]
Conversion rate per $1,000 principal amount	100
Conversion price, per share (in dollars per share) | $ / shares	$ 10.00
Common Stock Class A Non-voting | 2019 Convertible Notes
Debt Instrument [Line Items]
Conversion rate per $1,000 principal amount		62.2471
Conversion price, per share (in dollars per share) | $ / shares						$ 16.065
Additional Paid-in Capital | 2024 Convertible Notes
Debt Instrument [Line Items]
Carrying amount			$ 25,300,000