Employee Benefit Plans - Components of Net Periodic Benefit Income for the Pension Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 2,612	$ 2,133	$ 5,266	$ 4,290
Interest cost	22,071	24,099	44,184	48,231
Expected return on plan assets	(38,516)	(39,681)	(77,104)	(79,414)
Amortization of prior service credit	(37)	(87)	(74)	(175)
Amortization of actuarial loss	9,935	9,466	19,894	18,951
Net periodic benefit income	$ (3,935)	$ (4,070)	$ (7,834)	$ (8,117)