Employee Benefits - Pension Plan Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements, Recurring - USD ($) $ in Thousands		Dec. 31, 2019		Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	$ 116,652		$ 101,482
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	1,831	[1]	2,164	[3]
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[4]	101,320		91,830
Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	3,066	[5]	2,604	[6]
Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[7]	10,435		4,884
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	116,652		101,482
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	1,831	[1]	2,164	[3]
Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[4]	101,320		91,830
Level 1 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	3,066	[5]	2,604	[6]
Level 1 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[7]	10,435		4,884
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0		0
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	0	[1]	0	[3]
Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[4]	0		0
Level 2 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0	[5]	0	[6]
Level 2 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[7]	0		0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0		0
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[2]	0	[1]	0	[3]
Level 3 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[4]	0		0
Level 3 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0	[5]	0	[6]
Level 3 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[7]	$ 0		$ 0

[1] Refer to Note 1 Summary of Significant Accounting Policies, Fair Value Measurements for a description of levels within the fair value hierarchy.

[2] This asset class includes FDIC insured money market instruments.

[3] Rate of compensation increase is not applicable for 2019 and 2018 due to the amendment to freeze benefit accruals under the qualified and nonqualified defined benefit pension plans effective March 31, 2016.

[4] This asset class includes a variety of fixed income mutual funds which primarily invest in investment grade rated securities. Investment managers have discretion to invest in fixed income related securities including futures, options and other derivatives. Investments may be made in currencies other than the U.S. dollar.

[5] )The sole investment within this asset class is the Vanguard Total International Stock Index Fund Admiral Shares.

[6] The sole investment within this asset class is Harbor International Institutional Fund.

[7] This asset class includes individual domestic equities invested in an active all-cap strategy. It may also include convertible bonds.