Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 14,266	$ 15,824
Interest cost on benefit obligation	32,945	31,189
Expected return on plan assets	(43,792)	(45,149)
Amortization of:
Prior service cost	132	149
Net actuarial loss	9,731	7,761
Net periodic benefit cost	13,282	9,774
Portion of cost charged to expense	6,519	5,987
Other postretirement benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	3,937	4,346
Interest cost on benefit obligation	7,341	7,184
Expected return on plan assets	(9,122)	(9,147)
Amortization of:
Prior service cost	(9,471)	(9,492)
Net actuarial loss	946	1,561
Net periodic benefit cost	(6,369)	(5,548)
Portion of cost charged to expense	$ (3,126)	$ (1,788)