Qualified Employee Benefit Plans - Valuation of our Retirement Plan Assets by Pricing Observability Levels (Details) - Retirement Plan - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan, Assets [Abstract]
Total assets in the fair value hierarchy	$ 92,123	$ 76,433
Investments measured at net assets value	21,957	22,151
Total assets measured at fair value	114,080	98,584	$ 100,706
Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	64,805	54,078
Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	27,318	22,355
Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Cash
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	230	238
Cash | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	230	238
Cash | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Cash | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
U.S. Treasury Strips
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	27,318	22,355
U.S. Treasury Strips | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
U.S. Treasury Strips | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	27,318	22,355
U.S. Treasury Strips | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Fixed income mutual funds
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	19,518	18,362
Fixed income mutual funds | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	19,518	18,362
Fixed income mutual funds | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Fixed income mutual funds | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity mutual fund
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	33,875	26,508
Equity mutual fund | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	33,875	26,508
Equity mutual fund | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity mutual fund | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity securities
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	11,182	8,970
Equity securities | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	11,182	8,970
Equity securities | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity securities | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	$ 0	$ 0