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, 2017	Dec. 31, 2016	Dec. 31, 2015
Net amount at beginning of year	$ (110,744)
Taxes	(3,990)	$ 1,826	$ 1,913
Net amount arising during the period	11,355	(2,857)	(2,990)
Net reclassifications during the period	(15,889)	(3,058)	(3,889)
Net amount at end of year	(94,851)	(110,744)
Pension Plans [Member]
Net amount at beginning of year	120,627	128,244	134,261
Net (gain) loss	(14,874)	4,370	8,680
Taxes	3,869	(1,704)	(3,386)
Net amount arising during the period	(11,005)	2,666	5,294
Amortization of prior service (cost) credit	(288)	(518)	(605)
Amortization of actuarial loss	(14,484)	(16,339)	(17,937)
Taxes	5,761	6,574	7,231
Net reclassifications during the period	(9,011)	(10,283)	(11,311)
Net amount at end of year	100,611	120,627	128,244
OPEB [Member]
Net amount at beginning of year	(9,182)	(13,741)	(15,869)
Net (gain) loss	(471)	313	(3,777)
Taxes	121	(122)	1,473
Net amount arising during the period	(350)	191	(2,304)
Amortization of prior service (cost) credit	8,877	8,877	9,312
Amortization of actuarial loss	(1,537)	(1,717)	(2,047)
Taxes	(2,863)	(2,792)	(2,833)
Net reclassifications during the period	4,477	4,368	4,432
Net amount at end of year	$ (5,055)	$ (9,182)	$ (13,741)