Common Stock Incentive Plan - Restricted Stock Units (Details) - Restricted Stock Units - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Stock Incentive Plan
Vesting period	4 years	4 years
Remaining unamortized share-based compensation expense	$ 471
Number of shares
Outstanding nonvested shares, beginning of year	24,869	18,460	10,136
Shares granted	8,383	10,467	14,783
Shares vested	(10,118)	(4,058)	(6,459)
Shares forfeited	(8,166)	0	0
Outstanding nonvested shares, end of each period	14,968	24,869	18,460
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 55.97	$ 52.65	$ 52.21
Shares granted (in dollars per share)	49.96	60.56	52.76
Shares vested (in dollars per share)	55.01	52.70	52.21
Shares forfeited (in dollars per share)	53.45	0.00	0.00
Outstanding nonvested shares, end of each period (in dollars per share)	$ 54.62	$ 55.97	$ 52.65