Stock-Based Compensation Plans (Details 2) - Performance Shares	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Performance awards
Equity instruments outstanding at the beginning of the period (in shares)	251,163
Equity instruments granted (in shares)	81,708
Change in awards based on performance (in shares)	49,323
Converted to restricted stock (in shares)	(128,482)
Forfeited (in shares)	(1,529)
Equity instruments outstanding at the end of the period (in shares)	252,183
Weighted average grant date fair value per award
Equity instruments outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 136.74
Grant date fair value per share (in dollars per share) | $ / shares	176.59
Change in awards based on performance (in dollars per share) | $ / shares	119.26
Converted to restricted stock (in dollars per share) | $ / shares	118.75
Forfeited (in dollars per share) | $ / shares	158.40
Equity instruments outstanding at the end of the period (in dollars per share) | $ / shares	$ 155.27
Grants in period based on total shareholder metrics	49,374
Grants in period based on financial metrics	32,334