Benefit Plans - Summary of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Service cost	$ 3,233	$ 3,921	$ 6,021
Interest	953	1,036	1,387
Expected return on assets	(6)	(144)	(400)
Amortization of prior service costs	22	279	903
Recognition of actuarial items	963	673	(27)
Curtailment gain	(228)	0	0
Net periodic benefit cost	$ 4,937	$ 5,765	$ 7,884