Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 65,001	$ 53,816	$ 179,254	$ 145,441
Investment securities available for sale
Net unrealized gains (losses)	(1,986)	(22,894)	16,384	59,849
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	0	0	(14,738)	0
Amortization of net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	76	(1,441)	(2,499)	(4,465)
Reclassification adjustment for net gains (losses) realized in net income(1)	0	13	0	(6,201)
Income tax (expense) benefit	734	9,280	328	(18,768)
Other comprehensive income (loss) on investment securities available for sale	(1,176)	(15,042)	(525)	30,415
Defined benefit pension and postretirement obligations
Amortization of prior service cost	(38)	(80)	(113)	(55)
Amortization of actuarial loss (gains)	621	621	1,596	1,586
Income tax (expense) benefit	(225)	(206)	(567)	(584)
Other comprehensive income (loss) on pension and postretirement obligations	358	335	916	947
Total other comprehensive income (loss)	(818)	(14,707)	391	31,362
Comprehensive income	$ 64,183	$ 39,109	$ 179,645	$ 176,803