Schedule II - Valuation and Qualifying Accounts and Reserves (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Insurance And Claims Accruals [Member]
Insurance and claims accruals:
Balance at beginning of year		$ 13,998	$ 14,404	$ 13,838
Charged to costs and expenses		45,271	41,036	35,194
Deductions	[1]	(43,034)	(41,442)	(34,628)
Balance at end of year		16,235	13,998	14,404
Allowance for Doubtful Accounts [Member]
Insurance and claims accruals:
Balance at beginning of year		475	450	334
Charged to costs and expenses		59	264	225
Deductions	[2]	(229)	(239)	(109)
Balance at end of year		$ 305	$ 475	$ 450

[1] Claims payments

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