Employee Benefit Plans, Restricted Stock Plan (Details) - Restricted Stock [Member] - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Deferred Compensation Arrangements [Abstract]
Number of shares authorized (in shares)	150,000		150,000
Number of shares available for grant (in shares)	136,539		136,539
Unvested Shares [Roll Forward]
Outstanding, beginning of period (in shares)	8,691		8,783
Granted (in shares)	4,869		4,869
Forfeited (in shares)	0		0
Vested (in shares)	(3,626)		(3,718)
Outstanding, end of period (in shares)	9,934		9,934
Weighted Average Market Price [Roll Forward]
Outstanding, beginning of period (in dollars per share)	$ 51.19		$ 51.20
Granted (in dollars per share)	62.91		62.91
Forfeited (in dollars per share)	0		0
Vested (in dollars per share)	(50.54)		(50.58)
Outstanding, end of period (in dollars per share)	$ 57.18		$ 57.18
Additional General Disclosures [Abstract]
Share-based compensation expense	$ 63	$ 54	$ 119	$ 104
Compensation cost related to nonvested awards that has not yet been recognized	$ 568		$ 568
Period over which compensation cost is expected to be recognized			3 years