Retirement and Compensation Plans Fair Values of plan assets by category and hierarchy (Details) - USD ($) $ in Thousands		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		$ 361,336		$ 315,457
Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		101,693		81,569
Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		75,870		62,872
Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		183,773		171,016		$ 201,750
U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	26,294		20,138
U.S. equity funds | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	26,294		20,138
U.S. equity funds | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
U.S. equity funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
International funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	44,730		35,854
International funds | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	44,730		35,854
International funds | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
International funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Short-term fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	1,502		510
Short-term fixed income funds | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Short-term fixed income funds | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	1,502		510
Short-term fixed income funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	13,538		12,117
Fixed income funds | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Fixed income funds | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	13,538		12,117
Fixed income funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	17,941		13,788
U.S. equity funds | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
U.S. equity funds | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	17,941		13,788
U.S. equity funds | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Real estate fund
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	14,380		12,455
Real estate fund | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Real estate fund | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	14,380		12,455
Real estate fund | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[1]	0		0
Group annuity contract
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	157,248		148,106
Group annuity contract | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	0		0
Group annuity contract | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	0		0
Group annuity contract | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		157,248	[2]	148,106	[2]	181,403
Funded annuity contracts
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	10,378		10,500
Funded annuity contracts | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	0		0
Funded annuity contracts | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[2]	0		0
Funded annuity contracts | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		10,378	[2]	10,500	[2]	10,776
Corporate
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	28,509		24,002
Corporate | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	0		0
Corporate | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	28,509		24,002
Corporate | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	0		0
US Government Agencies Debt Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	30,648		25,039
US Government Agencies Debt Securities | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	30,648		25,039
US Government Agencies Debt Securities | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	0		0
US Government Agencies Debt Securities | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[3]	0		0
Limited partnerships
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[4]	16,147		12,410
Limited partnerships | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[4]	0		0
Limited partnerships | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[4]	0		0
Limited partnerships | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount		16,147	[4]	12,410	[4]	$ 9,571
Limited partnerships | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[4]	1,500		600
Defined Benefit Plan, Cash and Cash Equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[5]	21		538
Defined Benefit Plan, Cash and Cash Equivalents | Quoted prices in active markets for identical assets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[5]	21		538
Defined Benefit Plan, Cash and Cash Equivalents | Significant other observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[5]	0		0
Defined Benefit Plan, Cash and Cash Equivalents | Fair Value, Inputs, Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Plan Assets, Amount	[5]	$ 0		$ 0

[1] Represents mutual funds and pooled separate account investments with Principal Life Insurance Company.

[2] Represents annuity contracts with Farm Bureau Life

[3] Represents bonds to support the long duration fixed income investments.

[4] 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.

[5] Represents approximate fair value of cash held.