Retirement Plans - Rate and standard deviation assumptions - (Details) - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair value of pension plan assets
Purchases of shares of common stock	0	0	0
Pension:
Employee savings plan/ 401(K) plan	$ 7,381	$ 6,178	$ 5,873
Supplemental executive retirement plan	1,334	134	306
Post-retirement benefits	$ 251	$ 192	$ 212
Cash equivalents
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	3.00%
Standard Deviation Assumption (as a percent)	0.46%
High Grade Fixed Income
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	6.17%
Standard Deviation Assumption (as a percent)	3.42%
High Yield Fixed Income
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	8.37%
Standard Deviation Assumption (as a percent)	8.47%
International Fixed Income
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	6.53%
Standard Deviation Assumption (as a percent)	8.08%
Large Cap Equity
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	10.42%
Standard Deviation Assumption (as a percent)	13.26%
Mid Cap Equity
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	11.61%
Standard Deviation Assumption (as a percent)	15.75%
Mid/Small Cap Equity
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	10.46%
Standard Deviation Assumption (as a percent)	16.98%
Equity managers
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	7.57%
Standard Deviation Assumption (as a percent)	16.45%
Non-contributory defined benefit pension plan
Long-term rate of return and standard deviation assumptions used in developing long-term rate of return assumption for the pension plan
Rate of Return Assumption (as a percent)	7.75%	7.75%	7.75%
Fair value of pension plan assets
Fair value of pension plan assets	$ 31,387	$ 28,216	$ 27,444	$ 28,451
Estimated future benefit payments (including expected future service as appropriate)
2018	1,318
2019	1,382
2020	1,522
2021	1,604
2022	1,637
2023-2027	8,722
Estimated future benefit payments	16,185
Pension:
Expenses incurred and charged against operations	8,615	$ 6,299	$ 6,239
Non-contributory defined benefit pension plan | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	$ 31,387
Non-contributory defined benefit pension plan | Cash equivalents
Information about plan assets
Actual asset allocation (as a percent)	7.00%
Non-contributory defined benefit pension plan | Cash equivalents | Maximum
Information about plan assets
Allowable allocation percentages	35.00%
Non-contributory defined benefit pension plan | Cash equivalents | Minimum
Information about plan assets
Allowable allocation percentages	0.00%
Non-contributory defined benefit pension plan | Foreign equities
Fair value of pension plan assets
Fair value of pension plan assets	$ 3,743
Non-contributory defined benefit pension plan | Foreign equities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	$ 3,743
Non-contributory defined benefit pension plan | Equity managers
Information about plan assets
Actual asset allocation (as a percent)	64.00%
Non-contributory defined benefit pension plan | Equity managers | Maximum
Information about plan assets
Allowable allocation percentages	65.00%
Non-contributory defined benefit pension plan | Equity managers | Minimum
Information about plan assets
Allowable allocation percentages	55.00%
Non-contributory defined benefit pension plan | Fixed income
Information about plan assets
Actual asset allocation (as a percent)	29.00%
Non-contributory defined benefit pension plan | Fixed income | Maximum
Information about plan assets
Allowable allocation percentages	40.00%
Non-contributory defined benefit pension plan | Fixed income | Minimum
Information about plan assets
Allowable allocation percentages	20.00%
Non-contributory defined benefit pension plan | Money market funds
Fair value of pension plan assets
Fair value of pension plan assets	$ 2,146
Non-contributory defined benefit pension plan | Money market funds | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	2,146
Non-contributory defined benefit pension plan | Cash and cash equivalents
Fair value of pension plan assets
Fair value of pension plan assets	18
Non-contributory defined benefit pension plan | Cash and cash equivalents | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	18
Non-contributory defined benefit pension plan | Broad market fixed income
Fair value of pension plan assets
Fair value of pension plan assets	9,041
Non-contributory defined benefit pension plan | Broad market fixed income | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	9,041
Non-contributory defined benefit pension plan | U.S. equities
Fair value of pension plan assets
Fair value of pension plan assets	16,439
Non-contributory defined benefit pension plan | U.S. equities | Quoted Prices In Active Markets for Identical Assets (Level 1)
Fair value of pension plan assets
Fair value of pension plan assets	$ 16,439