Pension and Other Postretirement Employee Benefits (Change in Accumulated Other Comprehensive Loss (AOCL) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		$ 110,160	$ 113,128	$ 111,445	$ 114,503
Prior service credit (cost)	[1]	1,310	1,274	2,620	2,549
Actuarial loss	[1]	(2,292)	(2,857)	(4,887)	(5,507)
Total reclassification for the period	[1]	(982)	(1,583)	(2,267)	(2,958)
AOCI at end of period		109,178	111,545	109,178	111,545
Pension Plans [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		118,231	125,776	120,627	128,244
Prior service credit (cost)	[1]	(44)	(79)	(88)	(158)
Actuarial loss	[1]	(2,066)	(2,594)	(4,418)	(4,983)
Total reclassification for the period	[1]	(2,110)	(2,673)	(4,506)	(5,141)
AOCI at end of period		116,121	123,103	116,121	123,103
Other Postretirement Employee Benefits [Member]
Accumulated Other Comprehensive Income [Roll Forward]
AOCI at beginning of period		(8,071)	(12,648)	(9,182)	(13,741)
Prior service credit (cost)	[1]	1,354	1,353	2,708	2,707
Actuarial loss	[1]	(226)	(263)	(469)	(524)
Total reclassification for the period	[1]	1,128	1,090	2,239	2,183
AOCI at end of period		$ (6,943)	$ (11,558)	$ (6,943)	$ (11,558)

[1] Amortization of prior service credit (cost) and amortization of actuarial loss are included in the computation of net periodic cost (benefit).