Stock-Based Compensation Plans (Details)	6 Months Ended
	Jun. 30, 2016 $ / shares shares
Shares
Exercised (in shares)	(116,428)
2009 Plan | Stock Options
Shares
Options outstanding at the beginning of the period (in shares)	249,178
Exercised (in shares)	(56,774)
Forfeited (in shares)	0
Options outstanding at the end of the period (in shares)	192,404
Options exercisable at the end of the period (in shares)	192,404
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	113.78
Forfeited (in dollars per share) | $ / shares	0.00
Options outstanding at the end of the period (in dollars per share) | $ / shares	124.65
Options exercisable at the end of the period (in dollars per share) | $ / shares	$ 124.65
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
Options exercisable 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
Options exercisable at the end of the period (in dollars per share) | $ / shares	$ 77.91