Schedule II - Valuation and Qualifying Accounts and Reserves (Details) - Valuation and Qualifying Accounts and Reserves (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Insurance and Claims Accruals [Member]
Insurance and claims accruals:
Balance at beginning of year	$ 14,404		$ 13,838		$ 13,042
Charged to costs and expenses	41,036		35,194		32,548
Deductions	(41,442)	[1]	(34,628)	[1]	(31,752)	[1]
Balance at end of year	13,998		14,404		13,838
Allowance for Doubtful Accounts [Member]
Insurance and claims accruals:
Balance at beginning of year	450		334		470
Charged to costs and expenses	264		225
Deductions	(239)	[2]	(109)	[2]	(136)	[2]
Balance at end of year	$ 475		$ 450		$ 334

[1] Claims payments

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