Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 49,400	$ 46,365	$ 96,072	$ 91,564
Investment securities available for sale:
Net unrealized gains (losses)	(35,224)	35,557	12,194	56,184
Reclassification adjustment for net gains realized in net income	(1,242)	(34)	(1,242)	(412)
Income tax (expense) benefit	13,923	(13,655)	(4,182)	(21,441)
Other comprehensive income (loss) on investment securities available for sale	(22,543)	21,868	6,770	34,331
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	12	15	25	30
Amortization of actuarial losses	533	316	1,065	632
Income tax expense	(208)	(128)	(416)	(255)
Other comprehensive income on pension and postretirement obligations	337	203	674	407
Total other comprehensive income (loss)	(22,206)	22,071	7,444	34,738
Comprehensive income	$ 27,194	$ 68,436	$ 103,516	$ 126,302