Equity and redeemable securities (Narrative) (Details) (USD $)	Sep. 30, 2013	Aug. 14, 2013	Dec. 31, 2012
Class of Stock [Line Items]
Preferred stock par value (dollars per share)		$ 0.01
Preferred stock shares authorized (shares)	50,000,000	50,000,000
Preferred stock shares issued (shares)	0	0
Common stock, par value (dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized (shares)	300,000,000	300,000,000	300,000,000
Common stock shares issued (shares)	26,107,231	26,107,231	0
Common stock, shares, outstanding (shares)	26,107,231	26,107,231	0
Redeemable Class A Junior Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding (shares)	0	0
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, par value (dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized (shares)	22,725,500		22,725,500
Common stock shares issued (shares)	22,725,500		22,725,500
Common stock, shares, outstanding (shares)	0		11,590,005
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock, par value (dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized (shares)	3,246,500		3,246,500
Common stock shares issued (shares)	2,870,712		2,870,712
Common stock, shares, outstanding (shares)	0		2,870,712
Nonvested Stock [Member]
Class of Stock [Line Items]
Common stock shares issued (shares)	42,858	42,858