Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 57,901	$ 57,404	$ 52,011
Other comprehensive (loss) income, net of tax
Unrealized net holding gains (losses) arising during the year (pre-tax amounts of $12,757, $(6,660), and $7,438)	7,703	(4,021)	4,490
Reclassification adjustment for net gains related to securities available for sale included in net income (pre-tax amounts of $150, $3,274, and $144)	(90)	(1,977)	(86)
Amortization of prior service cost and actuarial gains (pre-tax amounts of $1,665, $1,767, and $2,581)	999	1,060	1,548
(Decrease) increase in unrecognized actuarial loss (pre-tax amounts of $(15,546), $(2,595), and $5,637)	(9,381)	(1,560)	3,415
Total other comprehensive (loss) income	(769)	(6,498)	9,367
Comprehensive income	$ 57,132	$ 50,906	$ 61,378