Qualified Employee Benefit Plans - Valuation of our Retirement Plan Assets by Pricing Observability Levels (Details) - Retirement Plan - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Assets [Abstract]
Total assets in the fair value hierarchy excluding net asset value investments	$ 76,433	$ 78,330
Investments measured at net assets value	22,151	22,376
Total assets measured at fair value	98,584	100,706	$ 86,699
Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	54,078	78,330
Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	22,355	0
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	238	91
Cash | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	238	91
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
US Treasury Strips
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	22,355
US Treasury Strips | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0
US Treasury Strips | Level 2
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	22,355
US Treasury Strips | Level 3
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0
Fixed income mutual funds
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	18,362	23,696
Fixed income mutual funds | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	18,362	23,696
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	26,508	29,352
Equity mutual fund | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	26,508	29,352
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	8,970	25,191
Equity securities | Level 1
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	8,970	25,191
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