Income taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Federal, State and Local Tax [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	$ 18,333	$ 24,853	$ 28,297
Increases in unrecognized tax benefits as a result of tax positions taken during the period	860	4,659
Decreases in unrecognized tax benefits because applicable returns are no longer subject to examination	(1,643)	(8,471)	(1,403)
Decreases in unrecognized tax benefits as a result of settlements with taxing authorities	(1,002)	(9,742)	(967)
Decreases in unrecognized tax benefits as a result of tax positions taken in prior years			(1,074)
Unrecognized tax benefits acquired in a business combination		7,034
Ending Balance	16,548	18,333	24,853
Accrued Interest [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	9,277	20,696	19,508
Increases in unrecognized tax benefits as a result of tax positions taken during prior years	4,514		11,468
Decreases in unrecognized tax benefits because applicable returns are no longer subject to examination	(1,412)	(8,201)	(670)
Decreases in unrecognized tax benefits as a result of settlements with taxing authorities		(5,497)	(549)
Decreases in unrecognized tax benefits as a result of tax positions taken in prior years		(1,645)	(9,061)
Unrecognized tax benefits acquired in a business combination		3,924
Ending Balance	12,379	9,277	20,696
Unrecognized Income Tax Benefits [Member]
Reconciliation Of Unrecognized Tax Benefits [Line Items]
Beginning Balance	27,610	45,549	47,805
Increases in unrecognized tax benefits as a result of tax positions taken during the period	860	4,659
Increases in unrecognized tax benefits as a result of tax positions taken during prior years	4,514		11,468
Decreases in unrecognized tax benefits because applicable returns are no longer subject to examination	(3,055)	(16,672)	(2,073)
Decreases in unrecognized tax benefits as a result of settlements with taxing authorities	(1,002)	(15,239)	(1,516)
Decreases in unrecognized tax benefits as a result of tax positions taken in prior years		(1,645)	(10,135)
Unrecognized tax benefits acquired in a business combination		10,958
Ending Balance	28,927	27,610	45,549
Less: Federal, state and local income tax benefits	(9,479)
Net unrecognized tax benefits if recognized, would impact the effective income tax rate	$ 19,448