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Commitments and Contingencies (Changes in Self-insurance Liability) (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Commitments and Contingencies Disclosure [Abstract]    
Balance, beginning of period $ 721,284 $ 739,029
Reserves provided 14,470 11,295
Payments (18,730) (14,194)
Balance, end of period $ 717,024 $ 736,130