Commitments and Contingencies (Details 2) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2013 item
Commitments and Contingencies
Deductible per claim for each line of coverage, until claim aggregate is reached	$ 1.0
Deductible per claim, once a policy's claim aggregate is reached per line of coverage	0.5
Deductible under health insurance benefits for qualified individuals	0.1
Number of multi-employer pension plans, the withdrawal of which could result in assessment of liabilities for additional contributions related to the underfunding of plans, minimum	1
Surety bonds
Commitments and Contingencies
Aggregate original face amount of outstanding bonds issued by the surety	944.8
Estimated remaining cost to complete bonded projects	$ 250.5