Share-based compensation LTIP Linked to TRS (Details) - LTIP linked to TRS - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation, Long Term Incentive Plan [Roll Forward]
Outstanding, beginning of period (in shares)	232,127	239,256	197,385
Outstanding, beginning of period (in dollars per share)	$ 32.88	$ 29.12	$ 25.94
Granted (in shares)	97,524	91,038	81,223
Granted (in dollars per share)	$ 22.95	$ 32.69	$ 30.71
Vested (in shares)	(70,189)	(87,753)	(35,397)
Vested (in dollars per share)	$ 35.46	$ 22.45	$ 14.85
Forfeited (in shares)	(1,506)	(10,414)	(3,955)
Forfeited (in dollars per share)	$ 28.32	$ 32.72	$ 30.82
Outstanding, end of period (in shares)	257,956	232,127	239,256
Outstanding, end of period (in dollars per share)	$ 28.45	$ 32.88	$ 29.12
Total weighted-average grant-date fair value	$ 2.2	$ 3.0	$ 2.5