Income taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	6 Months Ended	12 Months Ended
	Apr. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Contingency [Line Items]
Taxable income			$ (358.2)	$ 338.0
Federal income taxes paid net				118.0
Proceeds from income tax refunds	107.0
AMT Carryforward		6.8
Operating loss carryforwards		20.2
Number of years for cumulative pretax loss position		3 years
General business carry forward		2.5	3.7
Unrecognized tax benefits that would impact effective tax rate if recognized		1.4	1.5
State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
AMT Carryforward		6.8
Operating loss carryforwards		1,720.0	1,470.0
Domestic Tax Authority [Member]
Income Tax Contingency [Line Items]
State and local net operating loss carryforwards		115.7	70.4
Deferred income tax asset		40.5	24.6
State and local offsetting valuation allowance		6.8	7.0
Deferred income tax liability		17.9	17.0
Deferred Income Taxes [Member]
Income Tax Contingency [Line Items]
State and local deferred income tax asset		26.9	22.9
Foreign Tax Authority [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		68.7	65.5
Deferred income tax asset		17.9	17.0
Foreign tax credit carryforwards		3.2	3.2
State and local offsetting valuation allowance		1.6	1.6
Valuation Allowance [Member]
Income Tax Contingency [Line Items]
State and local offsetting valuation allowance		26.9	22.9
New York City [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		593.7	510.5
New York [Member]
Income Tax Contingency [Line Items]
State and local net operating loss carryforwards		593.9	511.1
New Jersey [Member]
Income Tax Contingency [Line Items]
State and local net operating loss carryforwards		512.2	424.8
Acquisition Line [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		31.3	31.3
Acquisition Line [Member] | State and Local Jurisdiction [Member]
Income Tax Contingency [Line Items]
Operating loss carryforwards		$ 24.3