Employee Benefit Plans - Components of Net Periodic Benefit Income for the Pension Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Retirement Benefits [Abstract]
Service cost	$ 2,584	$ 2,240	$ 7,850	$ 6,530
Interest cost	22,044	24,243	66,228	72,474
Expected return on plan assets	(38,470)	(38,061)	(115,574)	(117,475)
Amortization of prior service credit	(37)	(88)	(111)	(263)
Amortization of actuarial loss	9,920	9,549	29,814	28,500
Net periodic benefit income	$ (3,959)	$ (2,117)	$ (11,793)	$ (10,234)