Schedule II - Valuation and Qualifying Accounts and Reserves - Valuation Allowances and Reserves (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Insurance and Claims Accruals [Member]
Balance at Beginning of Year		$ 19,440	$ 16,235	$ 13,998
Charged to Costs and Expenses		53,609	49,614	45,271
Deductions	[1]	(46,872)	(46,409)	(43,034)
Balance at End of Year		26,177	19,440	16,235
Allowance for Doubtful Accounts [Member]
Balance at Beginning of Year		275	305	475
Charged to Costs and Expenses		33	88	59
Deductions	[2]	(8)	(118)	(229)
Balance at End of Year		$ 300	$ 275	$ 305

[1] Claims payments

[2] Write-off of bad debts, net of recoveries