Commitments And Contingencies (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	6 Months Ended		12 Months Ended
	Apr. 30, 2016	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	May 31, 2016
Cotter, Jr. Related Litigation Matters [Member]
Commitments And Contingencies [Line Items]
Litigation expense		$ 2.6	$ 1.2	$ 4.0
Coverage limit		$ 10.0
The STOMP Arbitration [Member]
Commitments And Contingencies [Line Items]
Settlement awarded	$ 2.3
Class B [Member] | Cotter Estate [Member] | Cotter, Jr. Related Litigation Matters [Member]
Commitments And Contingencies [Line Items]
Shares held by estate		427,808			327,808
Class B [Member] | Cotter Estate [Member] | Cotter, Jr. Related Litigation Matters [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Additional voting stock that may be awarded		25.50%
Class B [Member] | Cotter Trust [Member] | Cotter, Jr. Related Litigation Matters [Member]
Commitments And Contingencies [Line Items]
Shares held by estate		696,080			696,080
Voting stock percent		41.40%
Class B [Member] | Cotter Voting Trust [Member] | Cotter, Jr. Related Litigation Matters [Member]
Commitments And Contingencies [Line Items]
Voting stock percent		66.90%