Qualified Employee Benefit Plans - Valuation of our Retirement Plan Assets by Pricing Observability Levels (Details) - Retirement Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Assets [Abstract]
Total assets in the fair value hierarchy excluding net asset value investments	$ 78,330	$ 67,512
Investments measured at net assets value	22,376	19,187
Total assets measured at fair value	100,706	86,699	$ 86,292
Level 1 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	78,330	67,512
Level 2 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Level 3 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Cash [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	91	344
Cash [Member] | Level 1 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	91	344
Cash [Member] | Level 2 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Cash [Member] | Level 3 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Fixed Income Mutual Funds [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	23,696	21,441
Fixed Income Mutual Funds [Member] | Level 1 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	23,696	21,441
Fixed Income Mutual Funds [Member] | Level 2 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Fixed Income Mutual Funds [Member] | Level 3 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity Mutual Funds [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	29,352	25,037
Equity Mutual Funds [Member] | Level 1 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	29,352	25,037
Equity Mutual Funds [Member] | Level 2 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity Mutual Funds [Member] | Level 3 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity Securities [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	25,191	20,690
Equity Securities [Member] | Level 1 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	25,191	20,690
Equity Securities [Member] | Level 2 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	0	0
Equity Securities [Member] | Level 3 [Member]
Defined Benefit Plan, Assets [Abstract]
Total assets measured at fair value	$ 0	$ 0