Benefit Plans - Fair Value Measurement of Plan Assets (Details) - Pension Benefits - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Benefit plans
Fair value of plan assets	$ 99,581,000	$ 114,220,000	$ 108,550,000
Percentage of total	100.00%	100.00%
Minimum
Benefit plans
Percentage of assets in high-grade corporate bonds and U.S. government debt obligations	80.00%
Cash and cash equivalents
Benefit plans
Fair value of plan assets	$ 2,428,000	$ 1,942,000
Fixed income securities
Benefit plans
Percentage of total	68.70%	66.50%
U.S. Treasury securities
Benefit plans
Fair value of plan assets	$ 4,450,000	$ 4,533,000
Fixed income - U.S. government agency securities
Benefit plans
Fair value of plan assets	3,565,000	4,072,000
Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	53,721,000	59,786,000
Fixed income - municipal securities
Benefit plans
Fair value of plan assets	451,000	474,000
Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,272,000	7,044,000
Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	8,831,000	10,645,000
Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	10,482,000	13,048,000
Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	2,544,000	3,436,000
Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,176,000	1,618,000
Equity - international funds
Benefit plans
Fair value of plan assets	5,661,000	7,622,000
Level 1
Benefit plans
Fair value of plan assets	37,394,000	45,355,000
Level 1 | Cash and cash equivalents
Benefit plans
Fair value of plan assets	2,428,000	1,942,000
Level 1 | Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,272,000	7,044,000
Level 1 | Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	8,831,000	10,645,000
Level 1 | Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	10,482,000	13,048,000
Level 1 | Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	2,544,000	3,436,000
Level 1 | Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,176,000	1,618,000
Level 1 | Equity - international funds
Benefit plans
Fair value of plan assets	5,661,000	7,622,000
Level 2
Benefit plans
Fair value of plan assets	62,187,000	68,865,000
Level 2 | U.S. Treasury securities
Benefit plans
Fair value of plan assets	4,450,000	4,533,000
Level 2 | Fixed income - U.S. government agency securities
Benefit plans
Fair value of plan assets	3,565,000	4,072,000
Level 2 | Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	53,721,000	59,786,000
Level 2 | Fixed income - municipal securities
Benefit plans
Fair value of plan assets	451,000	474,000
Level 3
Benefit plans
Fair value of plan assets	$ 0	$ 0