Shareholders' Equity (Details Textual) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2018	Jan. 26, 2018	Jan. 31, 2016	Dec. 31, 2014
Shareholders' Equity (Textual)
Common stock authorized under repurchase program					$ 250,000,000	$ 170,000,000	$ 300,000,000	$ 250,000,000
Stock repurchases, Shares	2,400,000	1,700,000	2,700,000	2,400,000
Payments for stock repurchases	$ (125,400,000)	$ (100,000,000)	$ (120,100,000)	$ (125,400,000)
Common stock, shares authorized		120,000,000	120,000,000
Common Stock, Par or Stated Value Per Share		$ 0.625	$ 0.625
Common stock, shares issued		67,083,149	66,953,606
Common stock, shares outstanding		37,895,641.000000	39,442,893
Series A and B $2.50 Cumulative Convertible Preferred Stock [Member]
Shareholders' Equity (Textual)
Preferred stock, shares authorized		5,000,000
Preferred stock, par value		$ 1.00