Quarterly financial data - Condensed Consolidated Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011
Interim Reporting [Line Items]
Total assets	$ 472,897	$ 539,566	$ 530,395	$ 694,106	$ 685,380	$ 639,868	$ 632,860	$ 618,938
Total liabilities	105,065	193,599	205,196	381,734	393,306	347,777	352,884	354,949
Total shareholders' equity	367,832	345,967	325,199	312,372	292,074	292,091	279,976	263,989
Total liabilities and shareholders' equity	472,897	539,566	530,395	694,106	685,380	639,868	632,860	618,938
Previously Reported [Member]
Interim Reporting [Line Items]
Total assets	504,281	565,073	553,320	707,586	704,472	656,073	647,026	631,284
Total liabilities	105,183	187,463	199,104	369,178	389,301	345,130	349,116	354,201
Total shareholders' equity	399,098	377,610	354,216	338,408	315,171	310,943	297,910	277,083
Total liabilities and shareholders' equity	504,281	565,073	553,320	707,586	704,472	656,073	647,026	631,284
Adjustments [Member]
Interim Reporting [Line Items]
Total assets	(31,384)	(25,507)	(22,925)	(13,480)	(19,092)	(16,205)	(14,166)	(12,346)
Total liabilities	(118)	6,136	6,092	12,556	4,005	2,647	3,768	748
Total shareholders' equity	(31,266)	(31,643)	(29,017)	(26,036)	(23,097)	(18,852)	(17,934)	(13,094)
Total liabilities and shareholders' equity	$ (31,384)	$ (25,507)	$ (22,925)	$ (13,480)	$ (19,092)	$ (16,205)	$ (14,166)	$ (12,346)