Benefit Plans - Fair Value Measurement of Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Benefit plans
Fair value of plan assets	$ 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	$ 11,333	$ 4,274
Fixed income securities
Benefit plans
Percentage of total	61.80%	55.30%
U.S. Treasury securities
Benefit plans
Fair value of plan assets	$ 3,902	$ 8,299
U.S. government agency securities
Benefit plans
Fair value of plan assets	4,750	12,418
Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	51,322	12,279
Fixed income - municipal securities
Benefit plans
Fair value of plan assets	438	2,104
Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,678	11,515
Fixed income - exchange-traded funds
Benefit plans
Fair value of plan assets		2,721
Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	16,346	21,329
Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	3,605	9,036
Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,013
Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,420	4,334
Equity - international funds
Benefit plans
Fair value of plan assets	5,743	852
Level 1
Benefit plans
Fair value of plan assets	48,138	54,061
Level 1 | Cash and cash equivalents
Benefit plans
Fair value of plan assets	11,333	4,274
Level 1 | Fixed income - mutual funds
Benefit plans
Fair value of plan assets	6,678	11,515
Level 1 | Fixed income - exchange-traded funds
Benefit plans
Fair value of plan assets		2,721
Level 1 | Equity - large-cap mutual funds
Benefit plans
Fair value of plan assets	16,346	21,329
Level 1 | Equity - large-cap exchange-traded fund
Benefit plans
Fair value of plan assets	3,605	9,036
Level 1 | Equity - mid-cap exchange-traded funds
Benefit plans
Fair value of plan assets	3,013
Level 1 | Equity - small-cap exchange-traded funds
Benefit plans
Fair value of plan assets	1,420	4,334
Level 1 | Equity - international funds
Benefit plans
Fair value of plan assets	5,743	852
Level 2
Benefit plans
Fair value of plan assets	60,412	35,100
Level 2 | U.S. Treasury securities
Benefit plans
Fair value of plan assets	3,902	8,299
Level 2 | U.S. government agency securities
Benefit plans
Fair value of plan assets	4,750	12,418
Level 2 | Fixed income - U.S. corporate securities
Benefit plans
Fair value of plan assets	51,322	12,279
Level 2 | Fixed income - municipal securities
Benefit plans
Fair value of plan assets	$ 438	$ 2,104