Consolidated Statement of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Amortization of prior service credits, tax	$ 1,076	$ 186	$ 7,943
Curtailment gain, tax	0	0	3,788
Net actuarial loss (gain), tax	(9,039)	(8,524)	(43,732)
Settlement loss, tax	(2,312)	(8,213)	(7,050)
OPEB Plan amendments, tax	0	10,420	116,712
Other comprehensive gain (loss) before reclassification, tax	$ (26,360)	$ 24,232	$ (25,916)

Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Curtailment Gain (Loss), Tax

No definition available.

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- Definition

Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement Gain (Loss), Tax

+ References

No definition available.

+ Details

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- Definition

Other Comprehensive Income (Loss), Defined Benefit Plan, Plan Amendment, Tax

+ References

No definition available.

+ Details

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- Definition

Amount of tax expense (benefit) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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- Definition

Amount, before reclassification adjustment, of tax (expense) benefit for (increase) decrease in accumulated other comprehensive income for defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e640-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (i-k)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569616-111683

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- Definition

Amount of tax (expense) benefit for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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+ Details

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- References

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