Stock-Based Compensation - Assumptions (Details) - Stock options - shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Stock-Based Compensation
Awarded during the period (in shares)	126,260
Risk-free interest rates (as a percent)	2.80%
Risk-free interest rates, minimum (as a percent)		1.78%
Risk-free interest rates, maximum (as a percent)		2.09%
Expected life	5 years 4 months 17 days	5 years 6 months 22 days
Volatility (as a percent)	28.46%
Volatility, minimum (as a percent)		30.78%
Volatility, maximum (as a percent)		34.13%
Expected dividend yield (as a percent)	0.47%
Minimum
Stock-Based Compensation
Vesting period	3 years
Expected dividend yield (as a percent)		0.55%
Maximum
Stock-Based Compensation
Vesting period	4 years
Expected dividend yield (as a percent)		0.74%