Income Taxes - Components of Income Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Included in net income
Current federal	$ 11,195	$ 7,400	$ 20,707
Current state	1,953	1,961	600
Total current provision	13,148	9,361	21,307
Deferred federal	34,219	9,735	(10,676)
Deferred state	(1,754)	(1,032)	(926)
Total deferred provision	32,465	8,703	(11,602)
Total included in net income	45,613	18,064	9,705
Included in shareholder's equity
Deferred tax related to retirement benefits	3,788	(32,944)	(1,439)
Deferred tax related to securities	(1,583)	19,145	(606)
Deferred tax related to derivatives and hedging	(75)	(42)
Total included in shareholder's equity	$ 2,130	$ (13,841)	$ (2,045)