Fair Value - Annual Fair Value Measurements of Pension Plan Asses (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		$ 66,159	[1]	$ 65,189	$ 64,903
Mutual funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	[2]	41,002		29,071
U.S. agency securities
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		13,075		13,904
Corporate bonds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		9,171		7,010
Common stocks
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0		7,526
Common trust funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		2,911		7,678
Level 1
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		44,680		42,554
Level 1 | Mutual funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	[2]	41,002		29,071
Level 1 | U.S. agency securities
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		3,678		5,957
Level 1 | Corporate bonds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0		0
Level 1 | Common stocks
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0		7,526
Level 1 | Common trust funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0		0
Level 2
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		21,479		22,635
Level 2 | Mutual funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	[2]	0		0
Level 2 | U.S. agency securities
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		9,397		7,947
Level 2 | Corporate bonds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		9,171		7,010
Level 2 | Common stocks
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0		0
Level 2 | Common trust funds
Fair Value Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		$ 2,911		$ 7,678

[1] Additional information regarding the fair value of Pension Plan assets as of December 31, 2017 can be found in Note 21, "Fair Value."

[2] Includes mutual funds, money market funds, cash, cash equivalents, and accrued interest.