Significant Accounting Policies - Intangible Assets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Cost	$ 99.9	$ 24.8
Accumulated Amortization	(25.8)	(16.5)
In-place leases
Finite-Lived Intangible Assets [Line Items]
Cost	59.6	18.7
Accumulated Amortization	(18.6)	(11.8)
Permitted quarry rights
Finite-Lived Intangible Assets [Line Items]
Cost	18.0	0
Accumulated Amortization	(0.1)	0
Other
Finite-Lived Intangible Assets [Line Items]
Cost	22.3	6.1
Accumulated Amortization	$ (7.1)	$ (4.7)