Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 12,630	$ 13,990	$ 26,896	$ 29,814
Interest cost on benefit obligation	32,878	30,802	65,823	61,992
Expected return on plan assets	(43,161)	(44,467)	(86,953)	(89,616)
Amortization of:
Prior service cost	132	149	263	297
Net actuarial loss	10,627	7,767	20,358	15,528
Net periodic benefit cost	13,106	8,241	26,387	18,015
Portion of cost charged to expense	6,433	5,232	12,951	11,219
Other postretirement benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	3,560	4,068	7,497	8,413
Interest cost on benefit obligation	7,519	6,867	14,860	14,051
Expected return on plan assets	(9,125)	(9,281)	(18,247)	(18,428)
Amortization of:
Prior service cost	(9,471)	(9,492)	(18,942)	(18,984)
Net actuarial loss	1,349	880	2,295	2,441
Net periodic benefit cost	(6,168)	(6,958)	(12,537)	(12,507)
Portion of cost charged to expense	$ (3,027)	$ (2,482)	$ (6,153)	$ (4,271)