Share-based compensation (Details 3) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Weighted-average grant-date fair value per share
Total weighted-average grant-date fair value	$ 0.0	$ 0.1
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	252,302	264,326	261,235	288,151
Granted (in shares)	690	2,750	85,772	117,786
Vested (in shares)	(19,840)	(3,500)	(99,891)	(142,361)
Forfeited (in shares)	(14,316)	0	(28,280)	0
Outstanding, end of period (in shares)	218,836	263,576	218,836	263,576
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 28.35	$ 25.74	$ 25.77	$ 25.17
Granted (in dollars per share)	30.91	24.48	33.69	25.17
Vested (in dollars per share)	25.35	23.50	25.69	24.07
Forfeited (in dollars per share)	25.82	0.00	26.66	0.00
Outstanding, end of period (in dollars per share)	$ 28.79	$ 25.76	$ 28.79	$ 25.76
Total weighted-average grant-date fair value			$ 2.9	$ 3.0
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	163,423	257,956	257,956	232,127
Granted (in shares)	0	0	0	97,524
Vested (in shares)	0	0	(75,915)	(70,189)
Forfeited (in shares)	0	0	(18,618)	(1,506)
Outstanding, end of period (in shares)	163,423	257,956	163,423	257,956
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 27.63	$ 28.45	$ 28.45	$ 32.88
Granted (in dollars per share)	0.00	0.00	0.00	22.95
Vested (in dollars per share)	0.00	0.00	30.71	35.46
Forfeited (in dollars per share)	0.00	0.00	26.41	28.32
Outstanding, end of period (in dollars per share)	$ 27.63	$ 28.45	$ 27.63	$ 28.45
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 0.0	$ 2.2
LTIP awards linked to other performance conditions
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	220,158	359,782	364,731	296,843
Granted (in shares)	0	0	0	129,603
Vested (in shares)	0	0	(121,249)	(65,089)
Canceled (in shares)	(14,050)	0	(14,050)	0
Forfeited (in shares)	0	0	(23,324)	(1,575)
Outstanding, end of period (in shares)	206,108	359,782	206,108	359,782
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 26.00	$ 26.01	$ 26.01	$ 26.14
Granted (in dollars per share)	0.00	0.00	0.00	25.18
Vested (in dollars per share)	0.00	0.00	26.05	24.95
Canceled (in dollars per share)	26.89	0.00	26.89	0.00
Forfeited (in dollars per share)	0.00	0.00	25.85	26.07
Outstanding, end of period (in dollars per share)	$ 25.94	$ 26.01	$ 25.94	$ 26.01
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 0.0	$ 3.3