Stockholders' Equity - Shares of Common Stock Issued Under Employee Stock Purchase Plan (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Number of shares available, beginning balance (in shares)			213,760
Shares issued during period (in shares)	18,725	18,542	24,149
Number of shares available, ending balance (in shares)	152,344
Weighted average per share price of shares issued (in dollars per share)	$ 71.12	$ 55.40	$ 37.79