Stock-Based Compensation Plans (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Shares
Exercised (in shares)	(36,169)
2009 Plan | Employee and Directors Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	177,333
Exercised (in shares)	(25,426)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	151,907
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	108.88
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 126.83
1994 Plan | Employee and Directors Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	22,541
Exercised (in shares)	(10,743)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	11,798
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	99.47
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 58.28