Retirement Plans and Employee Stock Purchase Plans - Employee Stock Purchase Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Purchase Plans
Service period required to be completed for an employee to be eligible to participate in plan, minimum	90 days
Ownership interest in Company common stock to disqualify employee from participation in plan, maximum (as a percent)	5.00%
Maximum percentage of base salary which can be contributed by the employees	15.00%
Maximum amount of base salary which can be contributed annually by the employees	$ 25,000
Discount rate at which common stock can be purchased by the plan participant (as a percent)	5.00%
Summary of purchases made under the 2010 and 1995 Employee Stock Purchase Plans
Number of shares purchased	60,950	88,362	151,531
Average price of shares purchased (in dollars per share)	$ 49.85	$ 33.96	$ 20.87
2009 Stock Purchase Plan
Summary of purchases made under the 2010 and 1995 Employee Stock Purchase Plans
Stock based compensation expense	$ 0	$ 0	$ 0