Stock-based Compensation - Employee Stock Purchase Plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Shares purchased on the open market	106,994	100,199	97,734
Average per-share purchase price (in dollars per share)	$ 55.47	$ 51.60	$ 42.96
Total purchase price discount (in dollars)	$ 793,931	$ 739,598	$ 628,358