Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 85,929	$ 65,001	$ 244,578	$ 179,254
Investment securities available for sale
Net unrealized gains (losses)	(21,345)	(1,986)	(82,099)	16,384
Net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	0	0	0	(14,738)
Amortization of net unrealized gain (loss) on available for sale securities transferred to held to maturity securities	(52)	76	(684)	(2,499)
Reclassification adjustment for net losses (gains) realized in net income(1)	(30)	0	1,985	0
Reclassification from OCI due to change in accounting principle	0	0	(84)	0
Reclassification of certain tax effects from OCI	0	0	(8,419)	0
Income tax (expense) benefit	5,456	734	20,796	328
Other comprehensive income (loss) on investment securities available for sale	(15,971)	(1,176)	(68,505)	(525)
Defined benefit pension and postretirement obligations
Amortization of prior service cost	37	38	(112)	113
Amortization of actuarial loss (gains)	(551)	(621)	1,480	(1,596)
Reclassification of certain tax effects from OCI	0	0	(5,235)	0
Income tax (expense) benefit	174	225	(390)	567
Other comprehensive income (loss) on pension and postretirement obligations	340	358	(4,257)	916
Total other comprehensive income (loss)	15,631	818	72,762	(391)
Comprehensive income	$ 70,298	$ 64,183	$ 171,816	$ 179,645