Stock-Based Compensation Plans (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Shares
Exercised (in shares)	(41,123)
2009 Plan | Employee and Directors Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	177,333
Exercised (in shares)	(27,360)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	149,973
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 124.25
Exercised (in dollars per share) | $ / shares	110.47
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 126.77
1994 Plan | Employee and Directors Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	22,541
Exercised (in shares)	(13,763)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	8,778
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 77.91
Exercised (in dollars per share) | $ / shares	96.61
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 48.60