Significant Accounting Policies - Intangible Assets Acquired (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 2.1	$ 75.0
Weighted average useful life of finite-lived intangible assets acquired	1 year 9 months 18 days	9 years 10 months 24 days
In-place leases
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	2.1	51.3
Weighted average useful life of finite-lived intangible assets acquired	1 year 9 months 18 days	7 years 2 months 12 days
Permitted quarry rights
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	0	18.0
Weighted average useful life of finite-lived intangible assets acquired		19 years 0 months 0 days
Contract backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	0	2.6
Weighted average useful life of finite-lived intangible assets acquired		2 years 2 months 12 days
Trade name/customer relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets acquired	$ 0	$ 3.1
Weighted average useful life of finite-lived intangible assets acquired		8 years 0 months 0 days