Retirement Plans - Summary of Categories in Fair Value Hierarchy (Details 9) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	$ 286,539	$ 293,567
Equity Securities | U.S.
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	19,487	43,708
Equity Securities | non-U.S.
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	1,131	819
Equity Funds
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	1,314	28,052
Equity Funds | International LP
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets		28,052
Bond funds - government
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	3,126	22,237
Bond funds - other
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	231,710	150,712
Real estate
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	1,235	3,812
Cash and Cash Equivalents
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	11,145	7,823
Partnership
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	10,787	12,862
International hedge funds
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	6,604	23,542
Quoted Prices in Active Markets (Level 1)
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	31,763	52,350
Quoted Prices in Active Markets (Level 1) | Equity Securities | U.S.
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	19,487	43,708
Quoted Prices in Active Markets (Level 1) | Equity Securities | non-U.S.
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	1,131	819
Quoted Prices in Active Markets (Level 1) | Cash and Cash Equivalents
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	11,145	7,823
Significant Other Observable Inputs (Level 2)
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	236,150	201,001
Significant Other Observable Inputs (Level 2) | Equity Securities | non-U.S.
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Significant Other Observable Inputs (Level 2) | Equity Funds
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	1,314
Significant Other Observable Inputs (Level 2) | Equity Funds | International LP
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets		28,052
Significant Other Observable Inputs (Level 2) | Bond funds - government
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	3,126	22,237
Significant Other Observable Inputs (Level 2) | Bond funds - other
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	231,710	150,712
Significant Other Observable Inputs (Level 2) | Real estate
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	0	0
Significant Unobservable Inputs (Level 3)
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	10,787	12,862
Significant Unobservable Inputs (Level 3) | Partnership
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	10,787	12,862	$ 13,360
Significant Unobservable Inputs (Level 3) | International hedge funds
Fair values of pension plan assets
Defined Benefit Plan, Fair Value of Plan Assets	$ 0	$ 0