Employee Benefits and Stock Compensation - Employee Stock Purchase Plan (Details) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ESPP shares purchased	297,576	243,391	63,650
Employee Stock Option [Member]
ESPP shares purchased	12,434	10,483	9,083
Weighted-average employee purchase price	$ 22.67	$ 16.02	$ 13.99

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration