Stockholders' Equity- Shares of Common Stock Issued During 2016 under Employee Stock Purchase Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares Available, Beginning Balance	171,069			236,483
Shares Issued during Period (Shares)	18,542	24,149	22,723
Shares Available, Ending Balance	171,069			236,483
Shares Issued during Period (per Share Price)	$ 55.40	$ 37.79	$ 34.37