Benefit Plans - Summary of Change in Projected Benefit Obligations, in Plan Assets and Funded (Unfunded) Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in PBO:
PBO at beginning of year	$ 12,207	$ 70,417
Service cost	3,233	3,921	$ 6,021
Interest	953	1,036	1,387
Actuarial loss	1,307	1,361
Benefits paid by plan	(1,389)	(2,045)
Participants contributions	0	1,024
Curtailment gain	(228)	(18,474)
Settlements	0	(44,234)
Administrative expenses	0	0
Foreign exchange	196	(799)
PBO at end of year	16,279	12,207	70,417
Change in plan assets:
Fair value of assets at beginning of year	243	42,000
Actual return on assets	20	359
Employer contributions	103	1,369
Participant contributions	0	1,025
Benefits paid by plan	(69)	(449)
Administrative expenses	0	0
Settlements	0	(44,234)
Foreign exchange	0	173
Fair value of assets at end of year	297	243	$ 42,000
Unfunded status	15,982	11,964
Actuarial loss	871	849
Prior service cost	0	20
Amount recognized in AOCI, pre-tax	871	869
Accumulated benefit obligation	$ 8,161	$ 7,266