Stock-Based Compensation Plans (Details)	9 Months Ended
	Sep. 30, 2015 $ / shares shares
Shares
Exercised (in shares)	(155,173)
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	340,062
Exercised (in shares)	(54,719)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	285,343
Options exercisable at the end of the period (in shares)	224,246
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 122.67
Exercised (in dollars per share) | $ / shares	121.70
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	122.85
Options exercisable at the end of the period (in dollars per share) | $ / shares	$ 121.19
1994 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	272,402
Exercised (in shares)	(100,454)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	171,948
Options exercisable at the end of the period (in shares)	171,948
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 104.96
Exercised (in dollars per share) | $ / shares	90.82
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	113.23
Options exercisable at the end of the period (in dollars per share) | $ / shares	$ 113.23