Savings Plans, Pension Plans and Other Postretirement Employee Benefits (Change in Benefit Obligation, Plan Assets and Funded Status for Company-Sponsored Benefit Plans and Obligations) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 289,675
Fair value of plan assets at end of year	313,862	$ 289,675
Amounts recognized in the consolidated balance sheets [Abstract]
Noncurrent liabilities	(103,524)	(123,284)
Pension Plans [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	(385,461)	(382,071)
Service cost	(6,753)	(6,508)	$ (6,159)
Interest cost	(16,096)	(17,020)	(17,012)
Actuarial (loss) gain	(15,876)	(13,997)
Benefits paid	31,815	34,135
Benefit obligation at end of year	(392,371)	(385,461)	(382,071)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	289,675	292,200
Actual return on plan assets	49,158	28,626
Employer contributions and benefit payments	6,844	2,984
Benefits paid	(31,815)	(34,135)
Fair value of plan assets at end of year	313,862	289,675	292,200
Amounts recognized in the consolidated balance sheets [Abstract]
Current liabilities	(1,629)	(1,824)
Noncurrent liabilities	(76,880)	(93,962)
Funded status	(78,509)	(95,786)
OPEB [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	(33,337)	(35,471)
Service cost	(14)	(14)	(22)
Interest cost	(1,262)	(1,421)	(1,456)
Actuarial (loss) gain	471	(313)
Benefits paid	3,793	3,882
Benefit obligation at end of year	(30,349)	(33,337)	$ (35,471)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Employer contributions and benefit payments	3,794	3,882
Benefits paid	(3,794)	(3,882)
Amounts recognized in the consolidated balance sheets [Abstract]
Current liabilities	(3,705)	(4,015)
Noncurrent liabilities	(26,644)	(29,322)
Funded status	$ (30,349)	$ (33,337)