Consolidated Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 84,401	$ 86,136	$ 58,224
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	21,147	(52,949)	(4,648)
Reclassification adjustment for net gains included in income	(61)	(1)	(348)
Change in unamortized gain (loss) on available-for-sale investment securities transferred into held-to-maturity	(548)	0	56
Defined benefit post-retirement benefits plans:
Change in net actuarial loss	1,731	137	(77)
Other comprehensive income (loss), before tax	22,269	(52,813)	(5,017)
Deferred tax benefit (expense) related to other comprehensive income (loss)	(8,762)	20,781	1,974
Other comprehensive income (loss), net of tax	13,507	(32,032)	(3,043)
Total comprehensive income	$ 97,908	$ 54,104	$ 55,181