Retirement and Compensation Plans Level 3 plan assets changes in fair value (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period		$ 315,457
Fair Value of Plan Assets, End of period		361,336		$ 315,457
Significant unobservable inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period		171,016		201,750
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement		791		(20,460)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		7,140		8,726
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		4,826		(19,000)
Fair Value of Plan Assets, End of period		183,773		171,016
Group annuity contract
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period	[1]	148,106
Fair Value of Plan Assets, End of period	[1]	157,248		148,106
Group annuity contract | Significant unobservable inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period		148,106	[1]	181,403
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement		(1,066)		(21,353)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		5,382		7,056
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		4,826		(19,000)
Fair Value of Plan Assets, End of period	[1]	157,248		148,106
Funded annuity contracts
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period	[1]	10,500
Fair Value of Plan Assets, End of period	[1]	10,378		10,500
Funded annuity contracts | Significant unobservable inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period		10,500	[1]	10,776
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement		(730)		(896)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		608		620
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		0		0
Fair Value of Plan Assets, End of period	[1]	10,378		10,500
Limited partnerships
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period	[2]	12,410
Fair Value of Plan Assets, End of period	[2]	16,147		12,410
Limited partnerships | Significant unobservable inputs (Level 3)
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets beginning of period		12,410	[2]	9,571
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Purchase, Sale, and Settlement		2,587		1,789
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held		1,150		1,050
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Sold		0		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		0		0
Fair Value of Plan Assets, End of period	[2]	$ 16,147		$ 12,410

[1] Represents annuity contracts with Farm Bureau Life

[2] Represents interests in several limited partnerships. A limited partnership with a fair value estimate of $1.5 million as of December 31, 2019 and$0.6 million as of December 31, 2018, using net asset value per share as a practical expedient, has not been classified in the fair value hierarchy above in accordance with fair value reporting guidance.