Income taxes - Effective Income Tax Reconciliation (Detail)	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	35.00%		35.00%		35.00%		35.00%
federal income tax effect	(15.30%)		2.50%		(88.70%)		3.60%
Nondeductible charges (1)	(25.00%)	[1]	3.20%	[1]	(175.30%)	[1]	0.80%	[1]
Other, net	(3.20%)		(3.20%)		(11.70%)		(0.70%)
Actual income tax rate	(8.50%)		37.50%		(240.70%)		38.70%

[1] Nondeductible charges include certain professional fees related to the Mergers, reserve for legal proceedings, and meals and entertainment.