Common Stock Incentive Plan - Restricted Stock Units (Details) - Restricted Stock Units - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Stock Incentive Plan
Remaining unamortized share-based compensation expense	$ 935,000
Number of shares
Outstanding nonvested shares, beginning of year	18,460	10,136
Shares granted	10,467	14,783	10,136
Shares vested	(4,058)	(6,459)
Outstanding nonvested shares, end of each period	24,869	18,460	10,136
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 52.65	$ 52.21
Shares granted (in dollars per share)	60.56	52.76	$ 52.21
Shares vested (in dollars per share)	52.70	52.21
Outstanding nonvested shares, end of each period (in dollars per share)	$ 55.97	$ 52.65	$ 52.21
Four year vesting period | Service-based vesting | Minimum
Common Stock Incentive Plan
Vesting period	4 years	4 years
Five year vesting period | Service-based vesting | Maximum
Common Stock Incentive Plan
Vesting period	5 years	5 years