Stock-based Employee Compensation, Stock Option Plans, and Deferred Compensation Plan for Directors - Summary of Option Activity (Details)	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares
Number of Shares
Outstanding at beginning of period (in shares) | shares	1,114,169
Granted (in shares) | shares	260,000
Exercised (in shares) | shares	(46,055)
Expired/Forfeited (in shares) | shares	(7,500)
Outstanding at end of period (in shares) | shares	1,320,614
Exercisable at end of period (in shares) | shares	774,614
Weighted Average Exercise Price per share
Outstanding at beginning of period (in usd per share) | $ / shares	$ 52.40
Granted (in usd per share) | $ / shares	55.71
Exercised (in usd per share) | $ / shares	45.07
Expired/Forfeited (in usd per share) | $ / shares	56.07
Outstanding at end of period (in usd per share) | $ / shares	53.29
Exercisable at end of period (in usd per share) | $ / shares	$ 52.28
Aggregate Intrinsic Value
Outstanding at beginning of period | $	$ 543,662
Granted | $	0
Exercised | $	503,262
Expired/Forfeited | $	0
Outstanding at end of period | $	4,728,314
Exercisable at end of period | $	$ 3,640,270