Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 49,438	$ 50,207	$ 145,510	$ 141,771
Investment securities available for sale:
Net unrealized gains (losses)	22,907	(20,123)	35,101	36,061
Reclassification adjustment for net gains realized in net income	(2,796)	(57)	(4,038)	(469)
Income tax (expense) benefit	(7,725)	7,757	(11,907)	(13,684)
Other comprehensive income (loss) on investment securities available for sale	12,386	(12,423)	19,156	21,908
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	12	15	37	45
Amortization of actuarial losses	627	317	1,692	949
Income tax expense	(243)	(128)	(659)	(383)
Other comprehensive income on pension and postretirement obligations	396	204	1,070	611
Total other comprehensive income (loss)	12,782	(12,219)	20,226	22,519
Comprehensive income	$ 62,220	$ 37,988	$ 165,736	$ 164,290