Employee Benefit Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Officer cash bonus target minimum	35.00%
Officer cash bonus target maximum	135.00%
Officer cash bonus performance factor minimum	0.00%
Officer cash bonus performance factor maximum	200.00%
Other management cash bonus target minimum	6.00%
Other management cash bonus target maximum	30.00%
Outstanding stock options and warrants	655,822	602,115
Possible future share issuance authorized under equity incentive plans	2,404,131	2,718,296
Total shares reserved for future issuance	3,059,953	3,320,411
Possible future share issuance authorized under equity incentive plan restricted stock	800,000
Stock-based compensation expense	$ 6,692	$ 6,251	$ 6,884
Total unrecognized stock-based compensation costs	$ 5,200
Weighted average remaining term of vesting periods (in years)	2 years 2 months 20 days
401(k) Savings Plan [Abstract]
401(k) employer match percentage	75.00%
401(k) employer match salary limit percentage	6.00%
401(k) employer contribution	$ 1,400	1,300	1,300
Retirement Plan [Abstract]
Retirement plan qualification years of service (in years)	30 years
Retirement plan qualification age	55 years
Retirement plan qualification minimum years of service (in years)	10 years
Retirement plan immediate vesting percentage	100.00%
Deferred Compensation [Abstract]
Marketable securities of non-qualified deferred compensation plan fair value disclosure	$ 2,388	2,451	2,736
Non Qualified Deferred Compensation Obligation Fair Value Disclosure [Roll Forward]
Beginning deferred compensation liability	2,451	2,736	3,635
Mark to market adjustment to deferred compensation (in general and administrative expenses)	492	222	(32)
Distributions from deferred compensation plans	(555)	(507)	(867)
Ending deferred compensation liability	$ 2,388	$ 2,451	$ 2,736
Employee Stock Purchase Plan [Abstract]
Employee stock purchase plan salary limit percentage	25.00%
Employee stock purchase plan market percentage	85.00%
Number of days at end of quarter used to calculate ESPP discount	5 days
Shares of Common Stock issued under the employee stock purchase plans	33,278	27,773	29,496
Employee Stock Purchase Plan Market Percentage Discount	15.00%
Discount on newly issued shares expensed as additional compensation	$ 200	$ 200	$ 200
2014 Plan [Member]
Restricted Stock Activity Rollforward [Abstract]
Awarded and issued	34,669
Vested	(34,669)
2013 Plan [Member]
Restricted Stock Activity Rollforward [Abstract]
Awarded and issued		6,647
Vested		(6,647)
2012 Plan [Member]
Restricted Stock Activity Rollforward [Abstract]
Awarded and issued			61,860
Vested			(61,860)
Stock Options [Member]
Stock Option Activity Rollforward [Abstract]
Balance of options outstanding	587,115	687,228	577,321
Options granted	168,748	244,664	197,408
Options canceled	0	(14,743)	(3,829)
Options exercised	(115,041)	(330,034)	(83,672)
Balance of options outstanding	640,822	587,115	687,228
Stock Option Activity, Weighted Average Exercise Price Rollforward [Abstract]
Options outstanding, weighted average exercise price	$ 42.26	$ 37.97	$ 34.92
Options granted, weighted average exercise price	52.49	43.55	45.61
Options canceled, weighted average exercise price	0.00	42.64	40.21
Options exercised, weighted average exercise price	40.41	34.26	34.89
Options outstanding, weighted average exercise price	$ 45.29	$ 42.26	$ 37.97
Weighted average remaining life of outstanding options	7 years 8 months 15 days
Options exercisable	152,745
Weighted average exercise price	$ 41.39
Weighted average remaining life	6 years 4 months 25 days
Intrinsic value	$ 1,500
Cash received or receivable from options exercised	5,200	$ 13,400	$ 3,300
Total instrinsic value of options exercised	1,300	4,800	900
Total intrinsic value of options outstanding	$ 3,900	$ 5,100	$ 4,300
Prior to 2005 and after 2013 Stock Options [Member]
Retirement Benefits [Abstract]
Vesting term of stock options issued/Period for absolute total return of restricted stock issued (in years)	4 years
Option expiration period (in years)	10 years
Grant date fair value of each stock option granted (in dollars per option)	$ 6.72
Assumptions Used in Determining Fair Values of Options/Restricted Stock Granted:
Risk free interest rate	2.00%
Common stock dividend yield	3.40%
Expected volatility rate	19.50%
Average expected option life (in years)	5 years 9 months
Dividends paid for previous year	1 year
2005 to 2013 Stock Options [Member]
Retirement Benefits [Abstract]
Vesting term of stock options issued/Period for absolute total return of restricted stock issued (in years)	4 years
Option expiration period (in years)	7 years
Grant date fair value of each stock option granted (in dollars per option)		$ 4.61	$ 6.19
Assumptions Used in Determining Fair Values of Options/Restricted Stock Granted:
Risk free interest rate		1.40%	1.70%
Common stock dividend yield		3.90%	3.70%
Expected volatility rate		19.70%	22.40%
Average expected option life (in years)		5 years 9 months	5 years 9 months
Time-Based Restricted Stock [Member]
Retirement Benefits [Abstract]
Vesting term of stock options issued/Period for absolute total return of restricted stock issued (in years)	4 years
Restricted Stock Activity Rollforward [Abstract]
Balance of restricted stock outstanding	188,981	204,720	222,068
Awarded and issued	61,404	72,698	71,994
Vested	(78,139)	(84,212)	(85,809)
Forfeited	0	(4,225)	(3,533)
Balance of restricted stock outstanding	172,246	188,981	204,720
Restricted Stock Activity, Weighted Average Grant Date Fair Value Rollforward [Abstract]
Restricted stock outstanding, weighted average grant date fair value	$ 42.06	$ 39.74	$ 35.97
Awarded and issued, weighted average grant date fair value	52.49	43.59	45.91
Vested, weighted average grant date fair value	40.55	37.76	35.14
Forfeited, weighted average grant date fair value	0.00	41.96	39.94
Restricted stock outstanding, weighted average grant date fair value	46.46	42.06	39.74
Grant date fair value of restricted stock	$ 52.49	$ 43.59	$ 45.91
Fair value at grant date of restricted stock issued	$ 3,200	$ 3,200	$ 3,300
Vesting date fair value of restricted stock	$ 4,100	$ 3,700	$ 3,900
Total Return-Based Restricted Stock [Member]
Retirement Benefits [Abstract]
Vesting term of stock options issued/Period for absolute total return of restricted stock issued (in years)	3 years
Assumptions Used in Determining Fair Values of Options/Restricted Stock Granted:
Risk free interest rate	1.60%	0.90%	1.00%
Common stock dividend yield	3.50%	4.10%	3.80%
Expected volatility rate	42.80%	43.10%	43.00%
Dividends paid for previous year	1 year
Restricted Stock Activity Rollforward [Abstract]
Balance of restricted stock outstanding	183,724	195,303	207,957
Awarded and issued	84,013	64,701	118,817
Vested	(107,013)	(71,617)	(129,762)
Forfeited	0	(4,663)	(1,709)
Balance of restricted stock outstanding	160,724	183,724	195,303
Restricted Stock Activity, Weighted Average Grant Date Fair Value Rollforward [Abstract]
Restricted stock outstanding, weighted average grant date fair value	$ 39.82	$ 36.66	$ 35.70
Awarded and issued, weighted average grant date fair value	44.76	40.87	37.64
Vested, weighted average grant date fair value	37.88	36.50	31.97
Forfeited, weighted average grant date fair value	0.00	39.91	37.25
Restricted stock outstanding, weighted average grant date fair value	$ 44.72	$ 39.82	$ 36.66
Percentage of return-based restricted stock issued to officers that will vest, minimum	0.00%	0.00%	0.00%
Percentage of return-based restricted stock issued to officers that will vest, maximum	150.00%	150.00%	150.00%
Company total return, total return-based restricted stock	100.00%
Vesting percentage, total return-based restricted stock	75.00%	75.00%	75.00%
Grant date fair value of restricted stock	$ 44.76	$ 40.87	$ 37.64
Fair value at grant date of restricted stock issued	$ 2,400	$ 2,400	$ 2,500
Vesting date fair value of restricted stock	$ 5,600	$ 3,100	$ 5,900
Period preceding date of grant	3 months
Total Return-Based Restricted Stock Current Year Issuance [Member]
Restricted Stock Activity, Weighted Average Grant Date Fair Value Rollforward [Abstract]
Awarded and issued, weighted average grant date fair value	$ 49.59	$ 41.37	$ 43.77
Grant date fair value of restricted stock	$ 49.59	$ 41.37	$ 43.77