Debt (Details) - USD ($)	3 Months Ended			9 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Oct. 31, 2011
Debt
Proceeds from Lines of Credit					$ 140,000,000
Payments on the line of credit					(140,000,000)
Revolving line of credit
Debt
Maximum borrowing capacity		$ 75,000,000		$ 75,000,000
Unused commitment fee (as a percent)				0.06%
Outstanding amount		0		$ 0
Revolving line of credit | Maximum
Debt
Additional increase in borrowing capacity of the line of credit available at the entity's option		$ 75,000,000		$ 75,000,000
Revolving line of credit | One-month LIBOR
Debt
Debt, floating rate of interest basis				one-month LIBOR
Debt, floating rate of interest, basis spread (as a percent)				0.50%
Convertible Notes Due 2016
Debt
Debt aggregate principal value							$ 250,000,000
Stated percentage rate		1.00%		1.00%			1.00%
Conversion price (in dollars per share)		$ 131.24		$ 131.24			$ 47.69
Convertible debt, number of shares into which convertible							5,200,000
Amount by which aggregate conversion value exceeded par value				$ 55,400,000
Value of convertible note settled due to conversion requests		$ 2,900,000
Shares issued for conversion of convertible notes (in shares)		43,000
Number of common shares bought under note hedge		43,000
Gains (Losses) on Extinguishment of Debt		$ 60,000
Number of shares that could be issued for future conversions of outstanding convertible notes (in shares)		663,000
Interest expense incurred in connection with the entity's convertible notes
Contractual coupon rate of interest		$ 63,000	$ 582,000	390,000	1,832,000
Discount amortization		411,000	2,989,000	2,833,000	8,856,000
Interest expense-convertible notes		474,000	$ 3,571,000	3,223,000	$ 10,688,000
Carrying amount related to convertible notes
Principal balance		31,634,000		31,634,000		$ 138,750,000
Convertible debt, discount to reduce net carrying value		(1,617,000)		(1,617,000)		(12,336,000)
Discount, accumulated amortization		5,714,000		5,714,000		19,819,000
Carrying amount		$ 30,017,000		$ 30,017,000		$ 126,414,000
Subsequent Event | Convertible Notes Due 2016
Debt
Value of convertible note settled due to conversion requests	$ 26,000,000
Subsequent Event | Forecast | Convertible Notes Due 2016
Debt
Gains (Losses) on Extinguishment of Debt	$ 395,000