Stock-Based Compensation Plans 2 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Options
Exercised (in shares)	(52,936)	(12,546)	(66,458)
Options outstanding at the end of the period (in shares)	69,202
Additional disclosure
Cash proceeds from the exercise of stock options	$ 909,000	$ 235,000	$ 1,198,000
Tax benefit for the tax deduction from awards exercised	0	$ (581,000)	$ (877,000)
Unrecognized compensation cost	$ 1,059,000
Maximum
Additional disclosure
Percentage of target amount of performance shares	200.00%
Minimum
Additional disclosure
Percentage of target amount of performance shares	0.00%
Weighted Average [Member]
Additional disclosure
Expected recognition period for unrecognized compensation cost	1 year 6 months 30 days
Stock options
Number of Options
Options Outstanding at the beginning of the period (in shares)	136,560
Granted (in shares)	0	0	0
Exercised (in shares)	(52,936)
Forfeited or expired (in shares)	(14,422)
Options outstanding at the end of the period (in shares)	69,202	136,560
Options exercisable at the end of the period (in shares)	69,202
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 17.27
Exercised (in dollars per share)	17.18
Forfeited or expired (in dollars per share)	19.50
Options outstanding at the end of the period (in dollars per share)	16.87	$ 17.27
Options exercisable at the end of the period (in dollars per share)	$ 16.87
Additional disclosure
Weighted Average Remaining Contractual Term, Options outstanding	1 year 4 months 6 days
Weighted Average Remaining Contractual Term, Options Exercisable	1 year 4 months 6 days
Aggregate Intrinsic Value, Options outstanding	$ 1,167,405
Aggregate Intrinsic Value, Options Exercisable	1,167,405
Total intrinsic value of options exercised	1,718,000	$ 308,000	$ 1,457,000
Tax benefit for the tax deduction from awards exercised	(1,000,000)	$ (581,000)	(877,000)
Restricted Stock Units
Additional disclosure
Unrecognized compensation cost related to performance awards	$ 824,000
Number of Restricted/Performance Share Units
Restricted share units at the beginning of the period (in shares)	107,729
Granted (in shares)	41,755
Vested (in shares)	(40,695)
Forfeited (in shares)	(1,502)
Restricted share units at the end of the period (in shares)	107,287	107,729
Weighted Average Grant-Date Value
Restricted share units at the beginning of the period (in dollars per share)	$ 30.83
Granted (in dollars per share)	44.46
Vested (in dollars per share)	39.32
Forfeited (in dollars per share)	42.32
Restricted share units at the end of the period (in dollars per share)	$ 32.75	$ 30.83
Restricted Stock Units | Weighted Average [Member]
Additional disclosure
Expected recognition period for unrecognized compensation cost	1 year 4 months 10 days
Performance awards
Additional disclosure
Vesting period	3 years
Period to meet the performance goals	3 years
Number of Restricted/Performance Share Units
Restricted share units at the beginning of the period (in shares)	102,203
Granted (in shares)	31,558
Performance criteria adjustment (in shares)	9,332
Vested (in shares)	(45,214)
Restricted share units at the end of the period (in shares)	97,879	102,203
Weighted Average Grant-Date Value
Restricted share units at the beginning of the period (in dollars per share)	$ 35.25
Granted (in dollars per share)	44.10
Performance criteria adjustment (in dollars per share)	43.40
Vested (in dollars per share)	29.60
Restricted share units at the end of the period (in dollars per share)	$ 41.49	$ 35.25
GSWC
Additional disclosure
Tax benefit for the tax deduction from awards exercised	$ 0	$ (501,000)	(872,000)
GSWC | Stock options
Additional disclosure
Tax benefit for the tax deduction from awards exercised	$ (1,000,000)	$ (501,000)	$ (872,000)