Financing Arrangements (Details) $ / shares in Units, shares in Millions		1 Months Ended	3 Months Ended		9 Months Ended
	Apr. 12, 2017 USD ($) $ / shares shares	Mar. 31, 2016 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) shares	Sep. 30, 2017 USD ($) shares	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	$ 128,300,000
Convertible debt, equity portion	46,700,000
Long-term debt			$ 136,659,000		$ 136,659,000		$ 130,048,000
Amortization of Debt Discount (Premium)					5,988,000	$ 3,366,000
Payments for Repurchase of Equity, Prepaid Forward	$ 78,000,000				$ 0	$ 78,000,000
Treasury Shares Acquired, Estimated, Prepaid Forward | shares	9.2		9.0		9.0	6.0
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit agreement, current borrowing capacity		$ 250,000,000.0
Credit agreement, maximum borrowing capacity		300,000,000.0
Minimum Fixed Charge Coverage Ratio, minimum balance		$ 25,000,000.0
Minimum Fixed Charge Coverage Ratio, minimum percent		10.00%
Line of Credit Facility, Remaining Borrowing Capacity			$ 102,000,000		$ 102,000,000		$ 118,000,000
Amount outstanding			0		0
Convertible Debt [Member]
Line of Credit Facility [Line Items]
Debt Instrument	$ 175,000,000		175,000,000		175,000,000
Debt Instrument, Unamortized Discount			35,400,000		35,400,000
Interest rate	3.50%
Debt Instrument, Convertible, Conversion Ratio	94.0071
Convertible Debt Principal Amount Conversion	$ 1,000
Debt Instrument, Convertible, Conversion Price | $ / shares	$ 10.64
Effective rate	10.50%
Debt Issuance Costs, Net	$ 5,700,000
Percentage of conversion price of notes	130.00%
Percentage of trading price of notes	98.00%
Long-term debt			136,700,000		136,700,000
Interest Expense, Debt			1,500,000	$ 1,500,000	4,600,000	$ 2,800,000
Amortization of Debt Issuance Costs			200,000	200,000	600,000	400,000
Amortization of Debt Discount (Premium)			2,000,000	$ 1,800,000	6,000,000	$ 3,400,000
Long-term Debt [Member] | Convertible Debt [Member]
Line of Credit Facility [Line Items]
Debt Issuance Costs, Gross	$ 4,200,000
Debt Issuance Costs, Net			$ 2,900,000		$ 2,900,000
Additional Paid-in Capital [Member] | Convertible Debt [Member]
Line of Credit Facility [Line Items]
Debt Issuance Costs, Gross	$ 1,500,000
Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Unused Capacity, Commitment Fee Percentage		0.25%
Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Unused Capacity, Commitment Fee Percentage		0.375%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		1.50%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		2.00%
Base Rate [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		0.50%
Base Rate [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Basis Spread on Variable Rate		1.00%