Stockholder's Equity - Shares of Common Stock Issued During 2016 Under Employee Stock Purchase Plan (Details) - $ / shares	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Number of shares available, beginning balance (in shares)			171,069	171,069
Shares issued during period (in shares)	4,682	5,171	5,534
Number of shares available, ending balance (in shares)	155,682			155,682
Per share price of shares issued during period (in dollars per share)	$ 81.31	$ 58.48	$ 58.19