Capital Stock (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 28, 2016
Equity, Class of Treasury Stock [Line Items]
Treasury stock, common, shares		15,366,504	15,162,050	15,302,345
Preferred stock, par or stated value per share		$ 0.01
Preferred stock, shares outstanding		0	0	0
Purchase Of senior notes		$ 0	$ 87,271,000	$ 0
Share repurchase program [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares, acquired		0	0	0
Stock repurchase program, authorized amount		$ 100,000,000
Stock repurchase program, remaining authorized repurchase amount		$ 33,500,000
Employee Stock Purchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Preferred stock, shares authorized		1,000,000
Treasury stock, shares, acquired		415,418	234,901	292,168
Treasury stock issued during period		210,964	375,196	200,056
Restricted stock [Member]
Equity, Class of Treasury Stock [Line Items]
Treasury shares purchased		$ 3,200,000	$ 1,200,000	$ 2,300,000
Convertible Notes due 2017 [Member] | Convertible debt [Member] | Share repurchase program [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, authorized amount			78,100,000
Repurchased face amount					$ 11,200,000
Convertible Notes due 2017 [Member] | Senior notes [Member]
Equity, Class of Treasury Stock [Line Items]
Debt instrument, face amount		172,500,000.0
Convertible Notes due 2017 [Member] | Senior notes [Member] | Share repurchase program [Member]
Equity, Class of Treasury Stock [Line Items]
Purchase Of senior notes	$ 9,200,000
Convertible Notes due 2021 [Member] | Senior notes [Member]
Equity, Class of Treasury Stock [Line Items]
Debt instrument, face amount		$ 100,000,000	$ 100,000,000.0