Employee compensation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Weighted-average grant date fair value
Tax Basis					$ 0
Restricted Stock Awards
Restricted stock awards
Outstanding at the beginning of the period (in shares)	1,799,000		1,195,000		911,000
Granted (in shares)	1,234,000		1,469,000		932,000
Forfeited (in shares)	(148,000)		(229,000)		(251,000)
Vested (in shares)	(680,000)	[1]	(636,000)	[1]	(397,000)	[2]
Outstanding at the end of the period (in shares)	2,205,000		1,799,000		1,195,000
Weighted-average grant date fair value
Outstanding at the beginning of the period (in dollars per share)	$ 19.17		$ 15.06		$ 1.14
Fair value per performance share	$ 25.68		$ 18.17		$ 22.90
Forfeited (in dollars per share)	$ 22.56		$ 18.47		$ 15.61
Vested (in dollars per share)	$ 19.13	[1]	$ 18.69	[1]	$ 1.03	[2]
Outstanding at the end of the period (in dollars per share)	$ 22.63		$ 19.17		$ 15.06
Tax Benefit Realized from exercise of equity instruments other than options					0
Unrecognized equity and stock-based compensation expense	$ 27,600,000
Weighted average period over which unrecognized equity and stock-based compensation expense is expected to be recognized	1 year 5 months 16 days

[1] The vesting of certain restricted stock awards could result in federal and state income tax expense or benefit related to the difference between the market price of the common stock at the date of vesting and the date of grant. See Note 6 for additional discussion regarding the tax impact of vested restricted stock awards.

[2] Vestings in the year ended December 31, 2012 related to restricted stock awards converted in the Corporate Reorganization. Such shares have a tax basis of zero to the grantee and therefore result in no tax benefit to the Company.