Retirement Plans and Other Benefits - Net Periodic Benefit Costs and Portion including Portion Charged to Expense (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net periodic benefit costs and the portion of these costs charged to expense
Amortization of transition obligation	$ 0	$ 0
Pension Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	53,080	64,195	63,502
Interest cost on benefit obligation	129,194	112,392	119,586
Expected return on plan assets	(158,998)	(146,333)	(140,979)
Amortization of transition obligation			0
Amortization of prior service cost (credit)	869	1,097	1,143
Amortization of net actuarial loss	10,963	39,852	44,250
Net periodic benefit cost	35,108	71,203	87,502
Portion of cost charged to expense	21,985	38,968	36,333
Other Benefits
Net periodic benefit costs and the portion of these costs charged to expense
Service cost - benefits earned during the period	18,139	23,597	27,163
Interest cost on benefit obligation	41,243	41,536	46,467
Expected return on plan assets	(46,400)	(45,717)	(45,793)
Amortization of transition obligation	0	0	452
Amortization of prior service cost (credit)	(9,626)	(179)	(179)
Amortization of net actuarial loss	1,175	11,310	20,233
Net periodic benefit cost	4,531	30,547	48,343
Portion of cost charged to expense	$ 6,000	$ 18,469	$ 19,321