Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 69,456	$ 56,270
Investment securities available for sale
Net unrealized gains (losses)	(41,834)	3,152
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	0	(5,264)
Amortization of net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	(297)	(1,027)
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, before Tax	(84)	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI Reclass Tax Effect, before Tax, Total	(8,419)	0
Income tax (expense) benefit	10,635	1,195
Other comprehensive income (loss) on investment securities available for sale	(40,000)	(1,944)
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(38)	(38)
Amortization of actuarial loss (gains)	465	488
Other Comprehensive Income Loss Reclassification Adjustment From AOCI Defined Benefits Tax Effect BeforeTax	(5,235)	0
Income tax (expense) benefit	(107)	(171)
Other comprehensive income (loss) on pension and postretirement obligations	(4,915)	279
Total other comprehensive income (loss)	(44,915)	(1,665)
Comprehensive income	$ 24,541	$ 54,605