Equity Plans Equity Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Option Indexed to Issuer's Equity [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested, Grant Date Fair Value	$ 2,420	$ 1,913	$ 2,775
Document Period End Date	Dec. 31, 2019
Restricted Stock Awards, Performance-based, Vested Balance	7,676
Restricted Stock Awards, Performance-based, Outstanding Balance	42,700
Restricted Stock Awards, Performance-based, Issued, Intrinsic Value	6,120	52,629	15,390
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	$ 3,406	$ 2,117	$ 2,643
Proceeds from the exercise of stock options and similar instruments	$ 1,603	$ 1,371	$ 1,215
Employee Stock Purchase Plan [Abstract]
Shares issued during the period under the plan	30,255	25,907	27,743
Cash received from employees toward purchases under the plan	$ 1,682	$ 1,523	$ 1,282
Restricted Stock and Deferred Restricted Stock [Member]
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	3,123	$ 1,875	$ 2,429
Employee Stock Purchase Plan [Abstract]
Unrecognized compensation cost	$ 3,915
Deferred Restricted Stock Awards [Member]
Dividend Equivalent Rights [Abstract]
Cumulative dividend equivalent rights converted to deferred restricted stock awards	81,231	79,478	77,034
Dividend Equivalent Rights [Member]
Dividend Equivalent Rights [Abstract]
Liability for dividend equivalent rights	$ 93	$ 97	$ 139
Incentive Plan [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	1,800,000
Maximum number of shares per employee	600,000
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued Pursuant To The Plan	819,340	793,811	628,546
Shares issuable under the plan upon exercise of outstanding awards	174,764	124,275	228,885
Number of shares available for grant	805,896
CTWS Plan [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	156,022
Employee Stock Purchase Plan [Member]
Summary of Activity under the Plan [Abstract]
Compensation costs charged to income	$ 283	$ 242	$ 214
Proceeds from the exercise of stock options and similar instruments	$ 1,603	1,371	1,215
Employee Stock Purchase Plan [Abstract]
Purchase price of common stock to employees under the plan, percent	85.00%
Maximum amount of base salary employees can designate for share purchase under the plan	10.00%
Expenses recorded under the plan	$ 294	$ 265	$ 229
Unrecognized compensation cost	$ 206
Common Stock [Member] | Employee Stock Purchase Plan [Member]
Option Indexed to Issuer's Equity [Line Items]
Number of shares reserved for issuance under the plan	400,000