Employee Benefits - Pension Plan Assets Measured at Fair Value on Recurring Basis (Details) - Fair Value Measurements, Recurring - USD ($) $ in Thousands		Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	$ 101,482		$ 87,074
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[2]	2,164		1,780
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[3]	91,830		27,738
Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	2,604	[4]	4,016	[5]
Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[6]	4,884		53,540
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	101,482		85,294
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[2]	2,164		0
Level 1 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[3]	91,830		27,738
Level 1 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	2,604	[4]	4,016	[5]
Level 1 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[6]	4,884		53,540
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0		1,780
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[2]	0		1,780
Level 2 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[3]	0		0
Level 2 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0	[4]	0	[5]
Level 2 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[6]	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	[1],[2]	0		0
Level 3 | Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[3]	0		0
Level 3 | Equity index mutual funds—international
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1]	0	[4]	0	[5]
Level 3 | Domestic individual equities
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	[1],[6]	$ 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] 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.

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

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

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