Common Stock Incentive Plan - Restricted Stock Units (Details) - Restricted Stock Units - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Stock Incentive Plan
Vesting period (in years)	4 years	4 years	4 years
Remaining unamortized share-based compensation expense	$ 296
Number of shares
Outstanding nonvested shares, beginning of year	14,968	24,869	18,460
Shares granted	5,482	8,383	10,467
Shares vested	(4,939)	(10,118)	(4,058)
Shares forfeited	0	(8,166)	0
Outstanding nonvested shares, end of each period	15,511	14,968	24,869
Weighted average price
Outstanding nonvested shares, beginning of year (in dollars per share)	$ 54.62	$ 55.97	$ 52.65
Shares granted (in dollars per share)	69.58	49.96	60.56
Shares vested (in dollars per share)	54.90	55.01	52.70
Shares forfeited (in dollars per share)	0	53.45	0
Outstanding nonvested shares, end of each period (in dollars per share)	$ 59.82	$ 54.62	$ 55.97