Shareholders' equity - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	Mar. 20, 2015	Jul. 14, 2014	Dec. 19, 2013	Mar. 20, 2013	Mar. 19, 2013
Equity forward transaction
Public offering related to equity forward transaction (in shares)					6.1
Sale of common stock related to equity forward transaction (in dollars per share)					$ 26.75
Closing price of common stock (in dollar per share)					$ 27.01
Offering related to underwriters exercising their over-allotment option under equity forward transaction (in shares)				0.9
Shares borrowed by forward counterparty from third party					7.0
Underwriting discount (in dollars per share)					$ 1.00312
Forward sale price (in dollars per share)					25.74688
Initial fair value (in dollars per share)					$ 0
Delivery of net shares on settlement	4.7	1.0	1.3
Delivery of cash on settlement	$ 104.5	$ 23.9	$ 32.1
Underwriting discount	$ 4.7	$ 1.0	$ 1.3