Income taxes - NOL (Details) $ in Millions	Dec. 31, 2015 USD ($)
Operating loss carryforwards
Net operating loss carryforwards	$ 589.7
2027
Operating loss carryforwards
Net operating loss carryforwards	45.3
2028
Operating loss carryforwards
Net operating loss carryforwards	92.0
2029
Operating loss carryforwards
Net operating loss carryforwards	70.0
2030
Operating loss carryforwards
Net operating loss carryforwards	25.8
2031
Operating loss carryforwards
Net operating loss carryforwards	13.4
2032
Operating loss carryforwards
Net operating loss carryforwards	26.3
2033
Operating loss carryforwards
Net operating loss carryforwards	150.2
2034
Operating loss carryforwards
Net operating loss carryforwards	$ 166.7