Stock-Based Compensation Plans (Details)	9 Months Ended
	Sep. 30, 2016 $ / shares shares
Shares
Exercised (in shares)	(128,192)
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	249,178
Exercised (in shares)	(68,538)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	180,640
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 122.17
Exercised (in dollars per share) | $ / shares	116.37
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 124.37
1994 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	82,195
Exercised (in shares)	(59,654)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	22,541
Weighted average exercise price per share
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 103.27
Exercised (in dollars per share) | $ / shares	112.85
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 77.91