Earnings per share and shareholders' equity (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 20, 2015	Jul. 14, 2014	Dec. 19, 2013	Mar. 20, 2013	Mar. 19, 2013
Earnings per share, basic and diluted
Distributed earnings, Basic (dollars per share)		$ 0.31
Undistributed earnings, Basic (dollars per share)		0.14
Earnings per share, Basic (in dollars per share)	$ 0.31	$ 0.45
Equity forward transaction
Public offering related to equity forward transaction (in shares)							6,100,000
Sale of common stock related to equity forward transaction (in dollars per share)	$ 26.75						$ 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)						900,000
Shares borrowed by forward counterparty from third party	7,000,000
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			4,700,000	1,000,000	1,300,000
Delivery of cash on settlement			$ 104.5	$ 23.9	$ 32.1
Underwriting discount			$ 4.7	$ 1.0	$ 1.3
Stock appreciation rights (SARs) | Common stock
Antidilutive effect of HEI common stock
Antidilutive effects of SARs shares that were not included in the computation of diluted EPS (in shares)	0	102,000