Significant Accounting Policies - Intangible Assets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Cost	$ 101.0	$ 99.9
Accumulated Amortization	(37.1)	(25.8)
In-place leases
Finite-Lived Intangible Assets [Line Items]
Cost	61.6	59.6
Accumulated Amortization	(25.8)	(18.6)
Favorable leases
Finite-Lived Intangible Assets [Line Items]
Cost	16.6	16.6
Accumulated Amortization	(7.8)	(6.1)
Permitted quarry rights
Finite-Lived Intangible Assets [Line Items]
Cost	18.0	18.0
Accumulated Amortization	(0.7)	(0.1)
Contract backlog
Finite-Lived Intangible Assets [Line Items]
Cost	2.6	2.6
Accumulated Amortization	(2.5)	(1.0)
Trade name/customer relationships
Finite-Lived Intangible Assets [Line Items]
Cost	2.2	3.1
Accumulated Amortization	$ (0.3)	$ 0