Share-Based Compensation - Option Rollforward (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll [Forward]
Beginning Balance | shares	117,220
Options Granted | shares	42,477
Options Exercised | shares	(48,442)
Ending Balance | shares	111,255
Options Expected to Vest | shares	99,484
Options Exerciseable | shares	11,771
Weighted Average Exercise Price [Abstract]
Outstanding at beginning of year | $ / shares	$ 75.34
Options Granted | $ / shares	134.6
Options Exercised | $ / shares	75.94
Outstanding at End of Period | $ / shares	97.71
Options Expected to Vest | $ / shares	100.64
Options Exerciseable | $ / shares	$ 72.95
Share Based Compensation Arrangement By Share Based Payment Award Options Additional Disclosures [Abstract]
Weighted Average Remaining Contractual Term, Outstanding	5 years 4 months
Weighted Average Remaining Contractual Term, Expected to Vest	5 years 6 months
Weighted Average Remaining Contractual Term, Exercisable	3 years 11 months
Outstanding Options, Intrinsic Value | $	$ 6,013
Expected to Vest Options, Intrinsic Value | $	5,086
Exercisable Options, Intrinsic Value | $	$ 928