Savings Plans, Pension Plans and Other Postretirement Employee Benefits (Other Amounts Recognized in Consolidated Statements of Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net amount at beginning of year	$ (114,503)
Taxes	(1,826)	$ (1,913)	$ (15,598)
Net amount arising during the period	(2,857)	(2,990)	(24,396)
Net reclassifications during the period	(3,058)	(3,889)	19,672
Net amount at end of year	(110,744)	(114,503)
Pension Plans [Member]
Net amount at beginning of year	128,244	134,261	117,167
Net loss (gain)	4,370	8,680	43,223
Taxes	(1,704)	(3,386)	(16,857)
Net amount arising during the period	2,666	5,294	26,366
Amortization of prior service (cost) credit	(518)	(605)	(748)
Amortization of actuarial loss	(16,339)	(17,937)	(14,451)
Taxes	6,574	7,231	5,927
Net reclassifications during the period	(10,283)	(11,311)	(9,272)
Net amount at end of year	120,627	128,244	134,261
Other Postretirement Employee Benefits [Member]
Net amount at beginning of year	(13,741)	(15,869)	(18,447)
Net loss (gain)	313	(3,777)	(3,229)
Taxes	(122)	1,473	1,259
Net amount arising during the period	191	(2,304)	(1,970)
Amortization of prior service (cost) credit	8,877	9,312	9,641
Amortization of actuarial loss	(1,717)	(2,047)	(2,186)
Taxes	(2,792)	(2,833)	(2,907)
Net reclassifications during the period	4,368	4,432	4,548
Net amount at end of year	$ (9,182)	$ (13,741)	$ (15,869)