Employee Benefit Plans (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) installment $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Defined Benefit Plan Disclosure
Employer contribution for defined contribution plan	$ 3.2	$ 2.8	$ 2.0
Employee Stock Ownership Plan, common stock, shares outstanding (shares) | shares	139,436
Employee Stock Ownership Plan, common stock, fair value	$ 13.3
Outside Directors
Defined Benefit Plan Disclosure
Number of deferred common stock (shares) | shares	12,240	12,293	10,166
Weighted-average grant date fair values of deferred common stock (dollars per share) | $ / shares	$ 92.63	$ 101.34	$ 97.99
Nonqualified Deferred Compensation Plan
Defined Benefit Plan Disclosure
Deferred compensation expense	$ 0.3	$ 0.2	$ 0.1
Number of annual installments of deferred compensation (installment) | installment	2
Number of days after departure from board of directors for payment of deferred compensation (days)	90 days