Equity Plans Restricted Stock and Deferred Restricted Stock Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
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
Cash received from employees toward purchases under the plan	$ 1,682	$ 1,523	$ 1,282
Employee stock purchase plan, shares	30,255	25,907	27,743
Restricted Stock Awards, Performance-based, Issued, Intrinsic Value	6,120	52,629	15,390
Restricted Stock Awards, Performance-based, Vested Balance	7,676
Restricted Stock Awards, Performance-based, Outstanding Balance	42,700
Incentive Plan [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of shares reserved for issuance under the plan	1,800,000
CTWS Plan [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of shares reserved for issuance under the plan	156,022
Employee Stock Purchase Plan [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Expenses recorded under the plan	$ 294	$ 265	$ 229
Unrecognized compensation cost	$ 206
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%
Restricted Stock and Deferred Restricted Stock [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Unrecognized compensation cost	$ 3,915
Period for recognition of compensation cost	1 year 10 months 2 days
Restricted Stock and Deferred Restricted Stock [Member] | Incentive Plan [Member]
Restricted and Deferred Restricted Stock Awards Outstanding [Roll Forward]
Outstanding as of January 1, 2016, units	124,275
Issued, units	76,018
Exercised, units	25,529
Forfeited or expired	0
Outstanding as of December 31, 2016, units	174,764	124,275
Shares vested as of December 31, 2016, shares	81,611
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Weighted Average Grant Date Fair Value [Roll Forward] [Roll Forward]
Outstanding as of January 1, 2016, wtd avg grant date fair value	$ 38.80
Issued, wtd avg grant date fair value	59.78
Exercised, wtd avg grant date fair value	49.77
Forfeited or expired, wtd avg grant date fair value	0
Outstanding as od December 31, 2016, wtd avg grant date fair value	46.63	$ 38.80
Shares vested as of December 31, 2016, wtd avg grant date fair value	$ 31.12
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Nonvested as of January 1, 2016, units	55,098
Granted, units	76,018
Vested, units	(37,963)
Forfeited, units	0
Nonvested as of December 31, 2016, units	93,153	55,098
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Nonvested as of January 1, 2016, wtd avg grant date fair value	$ 54.42
Granted, wtd avg grant date fair value	59.78
Vested, wtd avg grant date fair value	59.27
Forfeited, wtd avg grant date fair value	0
Nonvested as of December 31, 2016, wtd avg grant date fair value	$ 39.57	$ 54.42
Restricted Stock and Deferred Restricted Stock [Member] | CTWS Plan [Member]
Restricted and Deferred Restricted Stock Awards Outstanding [Roll Forward]
Issued, units	105,233
Exercised, units	23
Forfeited or expired	2,279
Outstanding as of December 31, 2016, units	102,931
Deferred Restricted Stock Awards [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Cumulative dividend equivalent rights converted to deferred restricted stock awards	81,231	79,478	77,034
Dividend Equivalent Rights [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Liability for dividend equivalent rights	$ 93	$ 97	$ 139
Common Stock [Member] | Employee Stock Purchase Plan [Member]
Restricted and Deferred Restricted Stock Awards, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Number of shares reserved for issuance under the plan	400,000