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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of increase in benefit obligation of defined benefit plan from business combination.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Benefit Plans - Changes to Benefit Obligations and Fair Value of Plan Assets and Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Funded status and amounts recognized in balance sheets
Pension liabilities for underfunded plans				$ (134,218)	$ (132,904)
Pension Benefits
Changes to PBO or APBO
Benefit obligation at beginning of year	$ 209,407	$ 203,384
Service cost	629	944	$ 809
Interest cost	8,297	8,784	8,681
Actuarial (gain) loss	9,757	9,653
Benefit payments	(15,109)	(14,061)
Change due to the Reorganization Transactions		703
Benefit obligation at end of year	212,981	209,407	203,384
Changes to fair value of plan assets
Fair value of plan assets at beginning of year	108,550	89,161
Actual return on plan assets	13,623	1,348
Contributions		25,953
Benefit payments from trust	(7,953)	(7,912)
Fair value of plan assets at end of year	114,220	108,550	89,161
Funded status and amounts recognized in balance sheets
Pension assets for overfunded plans				14,008	11,618
Pension liabilities for underfunded plans				(112,769)	(112,475)
Funded status				(98,761)	(100,857)
PBO, ABO and fair value of plan assets
Projected benefit obligation	209,407	203,384	203,384	212,981	209,407
Accumulated benefit obligation				210,251	207,157
Fair value of plan assets	108,550	89,161	89,161	114,220	108,550
Overfunded (underfunded) portion of PBO/ABO				(98,761)	(100,857)
Pension Benefits \| Funded Plan
Changes to PBO or APBO
Benefit obligation at beginning of year	96,859
Interest cost	3,922	4,182	4,252
Benefit obligation at end of year	100,212	96,859
Changes to fair value of plan assets
Fair value of plan assets at beginning of year	108,550
Fair value of plan assets at end of year	114,220	108,550
PBO, ABO and fair value of plan assets
Projected benefit obligation	96,859	96,859		100,212	96,859
Accumulated benefit obligation				100,212	96,859
Fair value of plan assets	108,550	108,550		114,220	108,550
Overfunded (underfunded) portion of PBO/ABO				14,008	11,691
Pension Benefits \| Unfunded Plan
Changes to PBO or APBO
Benefit obligation at beginning of year	112,548
Benefit obligation at end of year	112,769	112,548
PBO, ABO and fair value of plan assets
Projected benefit obligation	112,548	112,548		112,769	112,548
Accumulated benefit obligation				110,039	110,298
Overfunded (underfunded) portion of PBO/ABO				(112,769)	(112,548)
Other Benefits
Changes to PBO or APBO
Benefit obligation at beginning of year	20,429	19,687
Service cost	738	697	734
Interest cost	776	812	770
Actuarial (gain) loss	(34)	(337)
Benefit payments	(460)	(430)
Benefit obligation at end of year	21,449	20,429	19,687
Funded status and amounts recognized in balance sheets
Pension liabilities for underfunded plans				(21,449)	(20,429)
Funded status				(21,449)	(20,429)
PBO, ABO and fair value of plan assets
Projected benefit obligation	$ 20,429	$ 19,687	$ 19,687	$ 21,449	$ 20,429