Incentive Plans (Additional Information about Stock Options Outstanding) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding, No. of Options	3,994,595
Options Outstanding, Avg. Life Remaining in Years	2 years 4 months 24 days
Options Outstanding, Aggregate Intrinsic Value | $	$ 3,719,263
Options Exercisable, No. of Options	3,182,345
Options Exercisable, Avg. Life Remaining in Years	1 year 9 months 18 days
Options Exercisable, Aggregate Intrinsic Value | $	$ 1,859,631
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	1,131,500
Options Outstanding, Avg. Life Remaining in Years	4 years 6 months
Options Outstanding, Aggregate Intrinsic Value | $	$ 3,719,263
Options Exercisable, No. of Options	319,250
Options Exercisable, Avg. Life Remaining in Years	4 years
Options Exercisable, Aggregate Intrinsic Value | $	$ 1,859,631
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	817,350
Options Outstanding, Avg. Life Remaining in Years	2 years 10 months 24 days
Options Exercisable, No. of Options	817,350
Options Exercisable, Avg. Life Remaining in Years	2 years 10 months 24 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	2,045,745
Options Outstanding, Avg. Life Remaining in Years	1 year
Options Exercisable, No. of Options	2,045,745
Options Exercisable, Avg. Life Remaining in Years	1 year