Stock-Based Compensation Plans (Details 4) - Performance awards - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Performance awards
Outstanding at the beginning of the period (in shares)	251,163	238,266	239,902
Granted (in shares)	81,708	94,054	85,636
Increase (decrease) in awards based on performance (in shares)	49,323	36,091	14,697
Converted to restricted stock (in shares)	(128,482)	(115,618)	(95,826)
Forfeited (in shares)	(1,942)	(1,630)	(6,143)
Outstanding at the ending of the period (in shares)	251,770	251,163	238,266
Weighted average grant date fair value per award
Outstanding at the beginning of the period (in dollars per share)	$ 136.74	$ 119.65	$ 95.20
Granted (in dollars per share)	176.59	141.92	148.49
Change in awards based on performance (in dollars per share)	119.26	101.52	78.50
Converted to restricted stock (in dollars per share)	118.75	94.67	78.50
Forfeited (in dollars per share)	159.39	141.98	110.34
Outstanding at the ending of the period (in dollars per share)	$ 155.25	$ 136.74	$ 119.65
Grants in period based on shareholder metrics (in shares)	49,374	61,039	55,162
Grants in period based on financial metrics (in shares)	32,334	33,015	30,474