Stock-Based Compensation Plans (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Shares
Exercised (in shares)	(66,949)
2009 Plan | Employee and Directors Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	124,212
Exercised (in shares)	(66,949)
Granted (in shares)	0
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	57,263
Options exercisable (in shares)	52,598
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 128.84
Exercised (in dollars per share) | $ / shares	129.06
Granted (in dollars per share) | $ / shares	0.00
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	128.58
Options exercisable (in dollars per share) | $ / shares	$ 125.70