Employee Benefit Plans - Schedule of Fair Value of Qualified Pension Plan Assets (Details) - Pension plans - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	$ 41,517	$ 42,061	$ 43,603
Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets		4,045
Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	40,776	37,527
Cash/cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	741	489
Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	741	489
Quoted prices in active markets for identical assets (Level 1) | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets		0
Quoted prices in active markets for identical assets (Level 1) | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	0	0
Quoted prices in active markets for identical assets (Level 1) | Cash/cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	741	489
Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	40,776	41,572
Significant observable inputs (Level 2) | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets		4,045
Significant observable inputs (Level 2) | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	40,776	37,527
Significant observable inputs (Level 2) | Cash/cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	0	0
Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	0	0
Significant unobservable inputs (Level 3) | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets		0
Significant unobservable inputs (Level 3) | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	0	0
Significant unobservable inputs (Level 3) | Cash/cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Fair value of qualified plan assets	$ 0	$ 0