Benefit Plans - Fair Value Measurement of Plan Assets (Details) - Pension Benefits - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit plans
Fair value of plan assets	$ 114,220	$ 108,550	$ 89,161
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	$ 1,942	$ 11,333
Fixed income securities
Benefit plans
Percentage of total	66.50%	61.80%
U.S. Treasury securities
Benefit plans
Fair value of plan assets	$ 4,533	$ 3,902
U.S. government agency securities
Benefit plans
Fair value of plan assets	4,072	4,750
Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	59,786	51,322
Fixed income - municipal securities
Benefit plans
Fair value of plan assets	474	438
Fixed income - mutual funds
Benefit plans
Fair value of plan assets	7,044	6,678
Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	10,645	16,346
Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	13,048	3,605
Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,436	3,013
Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,618	1,420
Equity - international funds
Benefit plans
Fair value of plan assets	7,622	5,743
Level 1
Benefit plans
Fair value of plan assets	45,355	48,138
Level 1 | Cash and cash equivalents
Benefit plans
Fair value of plan assets	1,942	11,333
Level 1 | Fixed income - mutual funds
Benefit plans
Fair value of plan assets	7,044	6,678
Level 1 | Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	10,645	16,346
Level 1 | Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	13,048	3,605
Level 1 | Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,436	3,013
Level 1 | Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,618	1,420
Level 1 | Equity - international funds
Benefit plans
Fair value of plan assets	7,622	5,743
Level 2
Benefit plans
Fair value of plan assets	68,865	60,412
Level 2 | U.S. Treasury securities
Benefit plans
Fair value of plan assets	4,533	3,902
Level 2 | U.S. government agency securities
Benefit plans
Fair value of plan assets	4,072	4,750
Level 2 | Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	59,786	51,322
Level 2 | Fixed income - municipal securities
Benefit plans
Fair value of plan assets	$ 474	$ 438