Stock-Based Compensation Plans (Details 4) - Performance awards - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Share Based Payment Award Maximum Measurement Period	3 years
Performance awards
Outstanding at the beginning of the period (in shares)	251,770	251,163	238,266
Granted (in shares)	100,965	81,708	94,054
Increase (decrease) in awards based on performance (in shares)	5,990	49,323	36,091
Converted to restricted stock (in shares)	(88,477)	(128,482)	(115,618)
Forfeited (in shares)	(3,119)	(1,942)	(1,630)
Outstanding at the ending of the period (in shares)	267,129	251,770	251,163
Weighted average grant date fair value per award
Outstanding at the beginning of the period (in dollars per share)	$ 155.25	$ 136.74	$ 119.65
Granted (in dollars per share)	155.31	176.59	141.92
Change in awards based on performance (in dollars per share)	148.79	119.26	101.52
Converted to restricted stock (in dollars per share)	148.79	118.75	94.67
Forfeited (in dollars per share)	160.33	159.39	141.98
Outstanding at the ending of the period (in dollars per share)	$ 157.21	$ 155.25	$ 136.74
Grants in period based on shareholder metrics (in shares)	62,043	49,374	61,039
Grants in period based on financial metrics (in shares)	38,922	32,334	33,015