Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 53,816	$ 49,438	$ 145,441	$ 145,510
Investment securities available for sale:
Net unrealized gains (losses)	(22,894)	22,907	59,849	35,101
Amortization of net unrealized gains on available for sale securities transferred to held to maturity securities	(1,441)	0	(4,465)	0
Reclassification adjustment for net (gains) losses realized in net income	13	(2,796)	(6,201)	(4,038)
Income tax (expense) benefit	9,280	(7,725)	(18,768)	(11,907)
Other comprehensive income (loss) on investment securities available for sale	(15,042)	12,386	30,415	19,156
Defined benefit pension and postretirement obligations:
Amortization of prior service cost	(80)	12	(55)	37
Amortization of actuarial loss	621	627	1,586	1,692
Income tax expense	(206)	(243)	(584)	(659)
Other comprehensive income on pension and postretirement obligations	335	396	947	1,070
Total other comprehensive income (loss)	(14,707)	12,782	31,362	20,226
Comprehensive income	$ 39,109	$ 62,220	$ 176,803	$ 165,736