Incentive Plans (Additional Information about Stock Options Outstanding) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, No. of Options	4,901,269
Options Outstanding, Avg. Life Remaining in Years	3 years
Options Outstanding, Aggregate Intrinsic Value | $	$ 11,652,680
Options Exercisable, No. of Options	3,197,269
Options Exercisable, Avg. Life Remaining in Years	1 year 10 months 24 days
Range of Exercise Prices per Option, $17.00 to $30.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices per option, lower limit | $ / shares	$ 17.00
Range of exercise prices per option, upper limit | $ / shares	$ 30.00
Options Outstanding, No. of Options	1,320,000
Options Outstanding, Avg. Life Remaining in Years	5 years 6 months
Options Outstanding, Aggregate Intrinsic Value | $	$ 11,652,680
Options Exercisable, Avg. Life Remaining in Years	0 years
Range of Exercise Prices per Option, $31.00 to $50.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices per option, lower limit | $ / shares	$ 31.00
Range of exercise prices per option, upper limit | $ / shares	$ 50.00
Options Outstanding, No. of Options	823,350
Options Outstanding, Avg. Life Remaining in Years	3 years 10 months 24 days
Options Exercisable, No. of Options	439,350
Options Exercisable, Avg. Life Remaining in Years	3 years 9 months 18 days
Range of Exercise Prices per Option, $51.00 to $65.00 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of exercise prices per option, lower limit | $ / shares	$ 51.00
Range of exercise prices per option, upper limit | $ / shares	$ 65.00
Options Outstanding, No. of Options	2,757,919
Options Outstanding, Avg. Life Remaining in Years	1 year 7 months 6 days
Options Exercisable, No. of Options	2,757,919
Options Exercisable, Avg. Life Remaining in Years	1 year 7 months 6 days