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	$ 58,224	$ 44,546	$ 37,356
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	(4,648)	17,168	(8,438)
Reclassification adjustment for net gains included in income	(348)	(1,544)	(170)
Unamortized premium on available-for-sale securities transferred into held-to-maturity	56	389	722
Defined benefit post-retirement benefits plans:
Change in the net actuarial loss	(77)	135	(940)
Other comprehensive income (loss), before tax	(5,017)	16,148	(8,826)
Deferred tax benefit (expense) related to other comprehensive income	1,974	(6,489)	3,126
Other comprehensive income (loss), net of tax	(3,043)	9,659	(5,700)
Total comprehensive income	$ 55,181	$ 54,205	$ 31,656