Share-based compensation (Details 3) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Weighted-average grant-date fair value per share
Total weighted-average grant-date fair value	$ 0.0	$ 0.0
Restricted stock units
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	261,691	332,158	261,235	288,151
Granted (in shares)	788	0	85,082	115,036
Vested (in shares)	(832)	(67,832)	(80,051)	(138,861)
Forfeited (in shares)	(9,345)	0	(13,964)	0
Outstanding, end of period (in shares)	252,302	264,326	252,302	264,326
Weighted-average grant-date fair value per share
Outstanding, beginning of period (in dollars per share)	$ 28.33	$ 25.04	$ 25.77	$ 25.17
Granted (in dollars per share)	31.25	0.00	33.72	25.19
Vested (in dollars per share)	26.60	22.31	25.77	24.09
Forfeited (in dollars per share)	28.36	0.00	27.52	0.00
Outstanding, end of period (in dollars per share)	$ 28.35	$ 25.74	$ 28.35	$ 25.74
Total weighted-average grant-date fair value			$ 2.9	$ 2.9
LTIP linked to TRS
Restricted stock awards and restricted stock units
Outstanding, beginning of period (in shares)	168,777	258,243	257,956	232,127
Granted (in shares)	0	731	0	97,524
Vested (in shares)	0	0	(75,915)	(70,189)
Forfeited (in shares)	(5,354)	(1,018)	(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	$ 28.46	$ 28.45	$ 32.88
Granted (in dollars per share)	0.00	22.95	0.00	22.95
Vested (in dollars per share)	0.00	0.00	30.71	35.46
Forfeited (in dollars per share)	27.42	26.50	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)	230,219	360,070	364,731	296,843
Granted (in shares)	0	730	0	129,603
Vested (in shares)	0	0	(121,249)	(65,089)
Forfeited (in shares)	(10,061)	(1,018)	(23,324)	(1,575)
Outstanding, end of period (in shares)	220,158	359,782	220,158	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	23.34	0.00	25.18
Vested (in dollars per share)	0.00	0.00	26.05	24.95
Forfeited (in dollars per share)	26.02	25.81	25.85	26.07
Outstanding, end of period (in dollars per share)	$ 26.00	$ 26.01	$ 26.00	$ 26.01
Total weighted-average grant-date fair value	$ 0.0	$ 0.0	$ 0.0	$ 3.3