Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 13,760	$ 14,266
Interest cost on benefit obligation	32,701	32,945
Expected return on plan assets	(43,710)	(43,792)
Amortization of:
Prior service cost (credit)	20	132
Net actuarial loss	12,489	9,731
Net periodic benefit cost	15,260	13,282
Portion of cost charged to expense	7,568	6,519
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,358	3,937
Interest cost on benefit obligation	7,565	7,341
Expected return on plan assets	(13,350)	(9,122)
Amortization of:
Prior service cost (credit)	(9,461)	(9,471)
Net actuarial loss	1,454	946
Net periodic benefit cost	(9,434)	(6,369)
Portion of cost charged to expense	$ (4,678)	$ (3,126)