Consolidated Statement of Comprehensive Income (Unaudited) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Other Comprehensive Income [Abstract]
Net income	$ 50,207	$ 45,658	$ 141,771	$ 140,934
Investment securities available for sale:
Net unrealized gains (losses)	(20,123)	(20,558)	36,061	(142,318)
Reclassification adjustment for net (gains) losses realized in net income	(57)	(248)	(469)	(582)
Income tax (expense) benefit	7,757	8,031	(13,684)	55,169
Other comprehensive income (loss) on investment securities available for sale	(12,423)	(12,775)	21,908	(87,731)
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	15	17	45	52
Amortization of actuarial losses	317	1,073	949	3,218
Income tax (expense) benefit	(128)	(420)	(383)	(1,262)
Other comprehensive income on pension and postretirement obligations	204	670	611	2,008
Total other comprehensive income (loss)	(12,219)	(12,105)	22,519	(85,723)
Comprehensive income (loss)	$ 37,988	$ 33,553	$ 164,290	$ 55,211