v3.3.1.900
Accounting for derivative instruments - AOCI (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
AOCI Attributable to Parent, Net of Tax      
Accumulated OCI balance at beginning of period   $ 0.2 $ (1.4)
Change in fair value of cash flow hedges $ (0.6) (1.0) 0.7
Realized from OCI during the period 0.8 0.9 0.9
Accumulated OCI balance at end of period     0.2
Gains expected to be realized from OCI in the next 12 months, net of tax     0.9
Interest rate swaps      
AOCI Attributable to Parent, Net of Tax      
Accumulated OCI balance at beginning of period 0.1 0.2 (1.5)
Change in fair value of cash flow hedges (0.6) (1.0) 0.7
Realized from OCI during the period 0.7 0.9 1.0
Accumulated OCI balance at end of period 0.2 0.1 0.2
Gains expected to be realized from OCI in the next 12 months, net of tax 0.8 0.9 0.9
Tax effect of gains (losses) expected to be realized from OCI in the next 12 months $ 0.6 $ 0.6 0.6
Natural gas swaps      
AOCI Attributable to Parent, Net of Tax      
Accumulated OCI balance at beginning of period     0.1
Realized from OCI during the period     $ (0.1)