Retirement Plans and Other Benefits - Net Periodic Benefit Costs and Portion including Portion Charged to Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	$ 59,627	$ 53,080	$ 64,195
Interest cost on benefit obligation	123,983	129,194	112,392
Expected return on plan assets	(179,231)	(158,998)	(146,333)
Amortization of prior service cost (credit)	594	869	1,097
Amortization of net actuarial loss	31,056	10,963	39,852
Net periodic benefit cost	36,029	35,108	71,203
Portion of cost charged to expense	20,036	21,985	38,968
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	16,827	18,139	23,597
Interest cost on benefit obligation	28,102	41,243	41,536
Expected return on plan assets	(36,855)	(46,400)	(45,717)
Amortization of prior service cost (credit)	(37,968)	(9,626)	(179)
Amortization of net actuarial loss	4,881	1,175	11,310
Net periodic benefit cost	(25,013)	4,531	30,547
Portion of cost charged to expense	$ (10,391)	$ 6,000	$ 18,469