Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 54,558	$ 57,901	$ 57,404
Other comprehensive (loss) income, net of tax
Unrealized net holding (losses) gains arising during the year (pre-tax amounts of $(2,471), $12,757, and $(6,660))	(1,492)	7,703	(4,021)
Reclassification adjustment for net gains related to securities available for sale included in net income (pre-tax amounts of $599, $150, and $3,274)	(362)	(90)	(1,977)
Amortization of prior service cost and actuarial gains (pre-tax amounts of $3,593, $1,665, and $1,768)	2,092	999	1,060
Increase in unrecognized actuarial loss and prior service cost (pre-tax amounts of $(24), $(15,546), and $(2,596))	(14)	(9,381)	(1,560)
Total other comprehensive income (loss)	224	(769)	(6,498)
Comprehensive income	$ 54,782	$ 57,132	$ 50,906