Retirement Plans - Summary of Fair Values of Pension Plan (Details 8) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair values of pension plan assets
Total fair value of plan assets	$ 286,539	$ 293,567
Equity Securities | U.S.
Fair values of pension plan assets
Total fair value of plan assets	19,487	43,708
Equity Securities | non-U.S.
Fair values of pension plan assets
Total fair value of plan assets	1,131	819
Equity Funds
Fair values of pension plan assets
Total fair value of plan assets	1,314	28,052
Equity Funds | International LP
Fair values of pension plan assets
Total fair value of plan assets		28,052
Bond funds - government
Fair values of pension plan assets
Total fair value of plan assets	3,126	22,237
Bond funds - other
Fair values of pension plan assets
Total fair value of plan assets	231,710	150,712
Real estate
Fair values of pension plan assets
Total fair value of plan assets	1,235	3,812
Cash and Cash Equivalents
Fair values of pension plan assets
Total fair value of plan assets	11,145	7,823
Partnership
Fair values of pension plan assets
Total fair value of plan assets	10,787	12,862
International hedge funds
Fair values of pension plan assets
Total fair value of plan assets	$ 6,604	$ 23,542