Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	$ 1,789	$ 6,992	$ 2,808	$ 7,611
Net periodic benefit cost	608	913	1,351	1,826
Impairment on available-for-sale securities		(780)		(876)	(1,414)	(1,409)	(3,927)	(24,795)
Interest income/(expense)	85,682	85,264	171,784	169,527
Tax (expense) or benefit	13,734	10,889	24,126	19,229
Net of tax	1,021	3,337	1,258	3,326
Accumulated Net Unrealized Gains and Losses on Available - for - Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	1,091	3,802	1,738	4,113
Accumulated Net Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	107	139	213	277
Accumulated Net Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax				144
Accumulated Defined Benefit Pensions Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	(177)	(604)	(693)	(1,208)
Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Net Unrealized Gains and Losses on Available - for - Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net securities gains	1,789	6,992	2,808	7,611
Impairment on available-for-sale securities		(780)		(876)
Total before tax	1,789	6,212	2,808	6,735
Interest income/(expense)	(698)	(2,410)	(1,070)	(2,622)
Net of tax	1,091	3,802	1,738	4,113
Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Net Unrealized Gains and Losses on Held-to-Maturity Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)	177	231	354	461
Tax (expense) or benefit	(70)	(92)	(141)	(184)
Net of tax	107	139	213	277
Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Net Gains and Losses on Cash Flow Hedges [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income/(expense)				240
Tax (expense) or benefit				(96)
Net of tax				144
Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Member] | Accumulated Defined Benefit Pensions Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net periodic benefit cost	(842)	(1,007)	(1,702)	(2,014)
Tax (expense) or benefit	665	403	1,009	806
Net of tax	$ (177)	$ (604)	$ (693)	$ (1,208)