Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Amortization of:
Portion of cost (credit) charged to expense	$ (12,859)	$ (6,095)
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	14,213	13,760
Interest cost on benefit obligation	31,007	32,701
Expected return on plan assets	(45,667)	(43,710)
Amortization of:
Prior service cost (credit)	0	20
Net actuarial loss	7,782	12,489
Net periodic benefit cost (credit)	7,335	15,260
Portion of cost (credit) charged to expense	2,242	7,568
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	5,105	4,358
Interest cost on benefit obligation	7,101	7,565
Expected return on plan assets	(10,520)	(13,350)
Amortization of:
Prior service cost (credit)	(9,461)	(9,461)
Net actuarial loss	0	1,454
Net periodic benefit cost (credit)	(7,775)	(9,434)
Portion of cost (credit) charged to expense	$ (5,605)	$ (4,678)