Stock-Based Compensation Plans (Details 2) - Performance Shares	9 Months Ended
	Sep. 30, 2018 $ / shares shares
Performance awards
Equity instruments outstanding at the beginning of the period (in shares)	251,770
Equity instruments granted (in shares)	100,965
Change in awards based on performance (in shares)	5,990
Converted to restricted stock (in shares)	(88,477)
Forfeited (in shares)	(2,309)
Equity instruments outstanding at the end of the period (in shares)	267,939
Weighted average grant date fair value per award
Equity instruments outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 155.25
Grant date fair value per share (in dollars per share) | $ / shares	155.31
Change in awards based on performance (in dollars per share) | $ / shares	148.79
Converted to restricted stock (in dollars per share) | $ / shares	148.79
Forfeited (in dollars per share) | $ / shares	158.52
Equity instruments outstanding at the end of the period (in dollars per share) | $ / shares	$ 157.23
Grants in period based on total shareholder metrics	62,043
Grants in period based on financial metrics	38,922